6o 



ORIGIN OF CHORDATES 



in. 3- 



mu. 



gen 



these are known only from the skeleton. The Pogonophora may also 

 be distantly related, their larva can be regarded as of tornaria type, 

 the coelom develops as in enteropneusts and the larval body shows 

 three parts, as does that of the adult in some species. 



Although it would be unwise to 

 suggest close relationship between the 

 polyzoans and the pterobranchs, the 

 similarities are sufficient to suggest that 

 the chordates arose from sedentary 

 creatures, feeding by means of ciliated 

 tentacles. The evidence is sufficiently 

 strong to encourage us to look for the 

 presence somewhere in the line of verte- 

 brate ancestry of an animal with this 

 habit. The difficulties of this view arise 

 when we come to consider how the fish- 

 like organization of a free-swimming 

 animal first appeared, a question better 

 dealt with after consideration of the 

 tunicates. 



4. Subphylum Tunicata. Sea squirts 



In the adult ascidians or sea squirts 

 there is no obvious trace of the fish-like 

 form at all. The majority of these 

 animals are sac-like creatures living on 

 the sea floor and obtaining their food 

 by ciliary action. Often the separate 

 individuals are grouped together to form 

 large colonies, but in Ciona intestinalis, 

 common in British waters, the indi- 

 viduals occur separately, and this is possibly the primitive con- 

 dition for the group. The whole of the outside of the body is covered 

 by a tunic, in which there are only two openings, a terminal mouth and 

 a more or less dorsal atriopore, both carried upon siphons (Fig. 32). 

 The tunic is made mainly of a carbohydrate, tunicin, closely related to 

 cellulose, with which is combined about 20 per cent, of glycoprotein. 

 It is secreted by the epidermis but contains special cells that have 

 arrived there by migration from the mesoderm. In some tunicates 

 calcareous secretions of various shapes are found in the tunic. The 

 mantle that lines the tunic is covered by a single-layered epidermis. 



Fig. 32. Diagram of structure of 

 Ciona. 



atr.p. atriopore ; e. endostyle ; gen.d. 

 genital duct; h. heart; int. intestine; 

 m. mouth; mu. muscle; oes. oeso- 

 phagus; ph. pharynx; st. stomach. 

 (After Berrill.) 



