58 ORIGIN OF CHORDATES in. z- 



highly specialized for their burrowing life, but showing through the 

 special features we can clearly see a plan that has similarity with both 

 the echinoderms and the chordates. The special value of study of these 

 animals is that it proves decisively that an affinity between these 

 groups exists. Exactly how they are all related is a more speculative 

 matter, which we shall deal with later (see p. 74). 



3. Class Pterobranchia 



These are small, colonial, marine, sedentary animals, which show 

 some signs of the general echinoderm-chordate plan of organization 

 we have been discussing. Cephalodiscus (Fig. 31) has been found on the 

 sea bottom at various depths, mainly in the southern hemisphere: 

 there are several species. The colony consists of a number of zooids 

 held together in a many-chambered gelatinous house. The zooids are 

 formed by a process of budding, but do not maintain continuity with 

 each other. Each zooid has a proboscis, collar, and trunk; there are 

 coeloms in each of these parts, and proboscis and collar pores. The 

 collar is prolonged into a number of ciliated arms, the lophophore, by 

 means of which the animal feeds. There is a large pharynx, opening 

 by a single pair of gill-slits, which serve as an outlet for the water 

 drawn in by the cilia of the tentacles for the purpose of bringing food. 

 The intestine is turned upon itself, so that the anus opens near the 

 mouth. A thickening in the roof of the pharynx corresponds exactly in 

 position with the stomochord and contains vacuolated cells. The blood 

 system consists of a series of spaces arranged on a plan similar to that 

 in Balanoglossus. There is a dorsal ganglion in the collar, but this is 

 not hollow. The gonads are simple sacs and development takes place 

 in the spaces of the gelatinous house. Gastrulation is by invagination 

 at least in some species and the coelom is formed as an enterocoele. 

 The larva somewhat resembles that of ectoproctous polyzoa, which 

 is not closely similar to the echinoderm larvae, but could be derived 

 from the same plan. 



Rhabdopleura occurs in various parts of the world, including the 

 North Atlantic and northern part of the North Sea. The zooids are 

 connected together and have proboscis, collar, and trunk, ciliated 

 arms, coelomic spaces with pores (not 'nephridia' as is sometimes 

 stated) and stomochord, but no gill-slit. The development is not 

 known. 



The Pterobranchia thus show undoubted signs of the enteropneust- 

 chordate plan of organization and provide also an interesting sug- 

 gestion of possible affinities with Polyzoa, Brachiopoda, and Phoronis. 





