90 PRIMITIVE ANIMALS 



The deep-sea organisms Cephalodiscus and Rhab- 

 dopleura are most nearly related to Balanoglossus. 

 Of a much more obscure nature is the Phylum 

 Brachiopoda, but mention may be made of them here 

 as they afford perhaps the best example of a persis- 

 tent type to be met with in the whole animal kingdom. 

 The Brachiopods have the body enclosed in a bivalve 

 shell, very closely resembling that of a Lamellibranch 

 Mollusc (e.g. a Cockle), with which group they were 

 originally classified, but their internal anatomy is 

 not built on the Molluscan type at all, so that the 

 resemblance of the shell in the two classes of animals 

 is certainly due to convergence and not to a common 

 ancestry. It is one of those instances of a close 

 resemblance in superficial structure which the mor- 

 phologist has continually to be on his guard against. 

 The internal anatomy of the Brachiopod is not very 

 simple ; it is a coelomate animal and it exhibits the 

 same tripartite division of the coelom which we met 

 with in the Echinoderm larvae and in Balanoglossus. 

 It is held on this and other grounds that the Brachio- 

 pods are obscurely related to the stock which has 

 given rise to Echinoderms, Balanoglossus and the 

 Chordates, and that they have nothing to do in 

 origin with the Appendiculata. There is a form of 

 Brachiopod called Lingida which burrows in the 

 sand between tide-marks or at a shallow depth and 

 is found in certain localities off the coasts of many 



