402 THE ANATOMY OF INVERTEBRATED ANIMALS. 



off from the alimentary canal, and are converted into the peri- 

 visceral csLvity, The latter, therefore, is an enterocoele. The 

 embryo elongates, and constrictions divide it into three seg- 

 ments, of which the anterior becomes fringed with long cilia, 

 and develops eye-spots. Thus the young Brachiopod acquires 

 a great resemblance to an ordinary Annelid larva. The re- 

 semblance is increased by the appearance of four bundles of 

 setag on the middle segment, which becomes produced into a 

 sort of hood, the free edges of which are at first turned back- 

 ward and bear these set^. As the larva grows, the third 

 segment becomes truncated at the end, and furnishes a sur- 

 face (provided with a shell gland ? i^ifrd), by which the larva 

 attaches itself. At the same time, the first, or prsestomial 

 segment, atrophies, and the setigerous hood developed from 

 the middle segment is retroverted, rapidly grows, and gives 

 rise to the lobes of the mantle, on which the valves of the 

 shell are developed. 



The resemblance of the larval Brachiopod to a Polyzoon, 

 and especially to Xoxosoma, is striking, and full}^ bears out 

 the conclusion as to the affinity of the Polyzoa with the 

 JBrachiopoda which results from the study of their adult 

 structure. On the other hand, the development of the Bra- 

 chiopoda no less strongly testifies to their close relations with 

 the Worms.^ 



In the course of the previous pages the terms dorsal and 

 ventral have been employed in the sense in which the^^ are 

 conventionally used by conchologists. But an interesting 

 question, and one not easy to settle, is. What relation do these 

 dorsal and ventral regions of a Brachiopod bear to the neural 

 and hajmal regions of a Polyzoon, or to those of a Lamelli- 

 branch, or of a Gasteropod ? 



If we compare one of the articulated Brachiopods, such as 

 Waldheiniia^ in its shell, with a polypide of a Cheilostoma- 

 tous Polyzoon in its cell, the dorsal valve will appear to an- 

 swer to the operculum, and the ventral valve to the cell. If 

 this comparison be just, the two lobes of the mantle of the 

 Brachiopod must both belong to the dorsal or hsemal aspect 

 of the body ; that which corresponds with the so-called dor- 

 sal valve of the shell being the anterior, and that which lines 



* The acceptance of the view originally propounded by Steenstrup, and so 

 ably urged by Prof. Morse, respecting the affinities of the Brachiopods with 

 the Worms (" Proceedings of Boston Society of Natural History," 1873), does 

 not to my mind weaken the opinion I have always held as to their affinities 

 with i\iQ Polyzoa, on the one hand, and with the higher J/oW««ca, on the other. 



