Cephalochordates (Branchiostoma) are unique in showing affinities with chordates as well as non-chordates.
[I] Non-chordate affinities
Cephalochordates have been regarded to be phylogenetically related to several non-chordate groups at one time or other. Only those with more important groups are being summarized below. But these can be overlooked in favour of the more chordate-like characteristics of cephalochordates.
Subphylum Cephalochordata . ( 106
1. Affinities with Annelida. Some of the common features are : (i) Body bilaterally symmetrical and metamerically segmented, (ii) metamerically arranged protonephridia with solenocytes (as in some polychaetes), (iii) well developed coelom, (iv) closed and similarly df<r«>'p1 blood * v c i c s\>t,m. and (v) filter in main blood vessels is in opposite dire i the two groups. Above all, the three basic 2 characters of Cephalochordata are not p* w ^ m m Annelida.
2. Affinities with Mollusca. It was Pallas (1778) who first described and named amphioxus as Umax lanceolatus considering it to be a slug.
But the ciliary mode of feeding and respiratory mechanism through water current which are common features of the two groups may be due to similar mode of life. Their anatomy is completely different. Moreover, molluscs are unsegmented and their locomotory podium is also unknown in eephalochordates.
3. Affinities with E c h i n o d e r m a t a . Echinoderms have asymmetrical body, enterocoelic coelom and similarly formed mesoderm.
Perforations in the calyx of some fossil echinoderms look similar to gill-slits of amphioxus. As in Branchiost oids have similar phosphagens (creatim 1. But all these similar features may uwau^w of a very remote common ancestry of the two groups.
t i l l C h o r d u e a f f i n i t i e s
lostoma) shows the three basic t b u i h i t !e .inns, viz. the notochord, dorsal tubui.tr i v m cuu- and pharyngeal gill-slits, in the most typical manner and there is no doubt about its chordate nature. However, it shows 1 tjor groups of phylum ( in the phylum remains uncertain.
1. k P f m i ' i i w i < l t h e o H e ' b o u u h \
Hemic h having
nurnerouj a n - bu«s, f i - f mechanism, (iii) respiratory mechanism, (iv) enterocoelic
v I k !! it/! i ,11 '[* * 1 1 H " 11 S K >.' >' • It V ,U b
uojeciwns. isttt, muscies in tiemienoraata are unsegmented, nervous system distinctly of
gill-slits dorsal in position and a postanal tail is lacking, moreover, inclusion of Hemichordata under Chordata is also uncertain because of doubtful nature of notochord. As such, Hemichordata, without question, are more primitive than Cephalochordata.
2. Affinities with U r o c h o r d a t a . Branchiostoma (Cephalochordata) and Herdmania (Urochordata) are regarded to be very closely related because of (i) primitive ciliary feeding and respiratory mechanisms, (ii) large pharynx bearing numerous lateral gill slits, epipharyngeal groove, endostyle and peripharyngeal bands, (iii) an ectodcrm-iin^ atr>al cavity opening to outside through at (atrial siphon), (iv) identical early stage: elastic cleavage, gastrulation by invagination) ot development, and (v) the ascidian larva having a continuous notochord, above it a dorsal hollow nerve cord, and a post-anal tail with median caudal fin without fin rays.
Objections. But the adult urochordates are extremely degenerate and sedentary animals having several features unrepresented in cephalochordates, such as (i) body unsegmented, (ii) covered by a test made of cellulose, (iii) with enterocoelic coelom, (iv) without notochord, and hollow nerve cord, (v) with a liver, (vi) a well-developed muscular heart covered by peritoneum, (vii) without nephridia, (viii) sexes united with hermaphrodite gonads and (ix) larva undergoing retrogressive metamorphosis to become the adult. These differences show that inspite of close similarities reflecting upon a probable common ancestry, the cephalochordates are better evolved than the urochordates.
Subphylum Cephalochordata . ( 89
3. Affinities with Cyclostomata. The Ammocoete larva of lamprey (Cyclostomata) and Branchiostoma show a striking similarity in many characters, such as : (i) elongated, slender fish-like body, (ii) continuous dorsal median fin, (iii) mouth surrounded by an oral hood and (iv) guarded by a velum, and (v) pharynx having endostyle and gill slits. Besides these fundamental chordate characters, their adults show metameric myotomes, persistent gill slits, velum and a postanal tail.
4. Affinities with other v e r t e b r a t e s . Besides cyclostomes, Branchiostoma also resembles other vertebrates in several ways, such as (i) meta-merically arranged myotomes, (ii) true coclom
iined by mesodermal epithelium, (iii) postanal tail, (iv) midgut diverticulum comparable with liver, (v) well-formed hepatic portal system and (vi) similar airangement of main longitudinal vessels wilh forward flow of blood in ventral and backward flow in dorsal blood vessel.
Objections. Cephalochordates differ f r o m cyclostomes and other vertebrates in most of their primitive features already described, such as (i) lack of head, paired limbs, skull, veriebral column, muscular heart, red blood corpuslces, brain, specialized sense organs, gonoducts, elc., and (ii) in possessing nephridia, atrium, numerous gonads, asymmetry etc.
[Ill] Systematic position
Cephalochordates {Branchiostoma) possess all the important chordate characters so that their inclusion in the phylum Chordata is beyond doubl and conclusive. However, their true systematic
placc in the phylum remains controversial. They are definitely more evolved than the hemichordates.
Wiley (1894) stated that Amphioxus (-Branchiostoma) is a prototype chordate winch can be placed along the main line of chordate evolution. According to Garstang (1928) and Berri]l (1958), cephalochordates and vertebrates boih evolved from a neotcnic ascidian larva which failed to metamorphose. But this view is no longer upheld. The specialized characters of Branchiostoma indicate that it is not on the direct line of evolution of chordates.
Costa (1834) and Yarre! (1836) and more recently Gregory (3936) consider Branchiostoma to be a modified and degenerate form of some jawless vertebrates (cyclostomes or Agnatha),
Some workers even considered Branchiostoma to be a permanent paedogeneiic larval form of some species of cyclostomes. But the absence of craniate or vertebrate characters (head, cranium, skull, vertebral column, special sense organs, etc.) and the presence of protonephridia unknown in chordates, show that it has a very primitive character.
Conclusion. Branchiostoma possesses a peculiar admixture of primitive, degenerate as well as specialized or secondary characters. As such, it can not be placed in the direct line of evolution of chordates. It is regarded to be a generalized and primitive type of chordate very close to the ancestral vertebrates. But, because of its many differences, it is not included in the vertebrates.
Instead it is placed in an independent subphylum Cephalochordata.
IMPORTANT QUESTIONS
» Long Answer Type Questions
I Enlist the primitive, degenerate and specialized characters of Cephalochordata.
2. Discuss the affinities and systematic position of Cephalochordata.
» Short Answer Type Questions
1. Enumerate general characters of Cephalochordata
Subphylum Cephalochordata . ( 108
» Multiple Choice Questions
1. M e m b e r s of s u b p h y l u m Cephalochordata live in : (a) Marine and shallow waters
(b) Marine and deep waters (c) S w a m p s
(d) Freshwater lakes
2. Cephalochordates are mostly sedentary and are buried ; (a) Completely under the bottom sand
(b) With only anterior end projecting above bottom sand (c) With only posterior and projecting above bottom sand (d) Under the bottom sand laterally
3. Cephalochordates are small and slender animal with : (a) Snake like and unsegmented body
(d) Metamerie and opaque body
(c) Fish like, metamerie and segmented body (d) Transparent, snake like body
4. Body of a Lancelet is divided into ;
(a) Head and tail (b) Head, trunk and tail (c) Head, neck, trunk and tail (d) Trunk and tail 5. Lance lets possess :
(a) Median fins, but paired appendages are absent (b) Paired appendages
(c) Paired appendages and operculum
(d) Paired appendages but median fins are absent 6. Exoskeleton in cephalochordates is :
(a) Well developed (b) Bony (c) Cartilaginous (d) Absent 7. Muscles in cephalochordates are :
(a) Dorso-ventral (b) Dorso-lateral (c) Ventro-lateral (d) Antero-posterior 8. Notochord in cephalochordates :
(a) Extends f r o m pharynx to tail (b) Extends from rostrum to pharynx (c) Extends f r o m rostrum to tail (d) Completely absent
9. In cephalochordates, gill slits open into :
(a) Pharynx (b) Atrium (c) Coelom (d) Heart
10. In cephalochordates respiration takes place : (a) Through gills
(b) Through nostrils
(c) Through general body surface (d) Through limes
11. Circulatory system in »ephilochotdates is ;
(a) Well devv" *peo c h i t J , with a heart and lacks a respirator} r. „
(b) Poorly developed, open, with a heart and lacks a respiratory pigment
(c) Well developed, open, with a heart and lacks a respiratory pigment
(d) •Well develooed, d o s e d , without a heart and lacks a mem
12. s, excretion takes place by : with solenocytes
with solenocytes
„ v v-,ik i„'phmi<\ without solenocytes without solenocytes 13. ... „vphalochordates is :
(a) Dorsal, tubular with ganglia and brain (b) Dorsal, tubular without ganglia and brain (c) Ventral, tubular with ganglia and brain (d) Ventral, tubular without ganglia and brain 14. In cephalochordates i
(a) Sexes are separate
(b) Organisms are hermaphrodite (c) One pair of gonads are present
(d) O r g a n i s m s are hermaphrodite, with metamerically arranged gonads
15. Development in Cephalochordata is : (a) Indirect including a parasitic larva (b) Direct including a parasitic larva (c) Indirect including a free swimming larva (d) Indirect including a free s w i m m i n g larva
ANSWERS
1. ( a ) 2. ( b ) 3 . ( c ) 4 . (cl) 5. (a) 6, ( d ) 7 . (b) 8. ( c ) 9. ( b ) 10. ( c ) 11. (d) 12, ( a ) 13. ( b ) 14. (a) 15. ( c ) .