THE BRACHIOPODS. 



445 



of greatly enlarged cirrate appendages, comparable with those of brachiopods, 

 or as the laterally extended row of many minute appendages. 



It will also be observed that the entoprocta are attached by stalk-like out- 

 growths from the haemal surface of the head, in typical acraniate fashion (Fig. 

 301, J), while the adult ectoprocta of the second generation, like Phoronis, are 

 attached by the evaginated neural surface. (Fig. 303, A.) 



VII. The Brachiopods. 



Our knowledge of the development of the brachiopods is fragmentary and 

 the data we do possess are lacking in detail and precision. The evidence, so far 

 as it goes, indicates that the brachiopods belong with the acraniates and that 

 their structure is best interpreted as a modification of the arthropod type. In 

 fact, they appear to have retained some of the characteristic features of the cirri- 

 peds in a less modified form than any other group of acraniates, while their own 

 distinctive features constitute a natural transition to the condition realized in the 



Trt 



Fig. 304. — Diagrams of a brachiopod. A, Seen in optical section; B, from the neural surface. 



phoronida and polyzoa. As in cirripeds, the eggs pass through their early stages 

 of development, in some cases the whole larval development (Stringocephalus, 

 Zittel) in brood pouches formed by folds in the mantle chamber. 



Cleavage is total and almost equal, giving rise to a hollow blastula that is in- 

 folded to form a telocoele. The telopore closes, the mesoderm separates from the 

 entoderm as two coelomic chambers, and the body is divided by two transverse 

 constrictions into what appear to represent the cephalic, thoracic, and abdominal 

 regions. 



The details of the method of attachment and of the subsequent metamorphosis' 

 are not clearly understood, but they appear to be essentially the same as in Pedicel- 

 lina. A haemal outgrowth serves for the permanent attachment of the larva and 

 a voluminous mantle fold encloses the body in a typical atrial chamber. The 

 mantle secretes a thick shell, resembling in its somewhat complex minute structure, 

 the simpler forms of the mantle skeleton in the cirripeds. 



The two valves are usually spoken of as dorsal and ventral, but morpholog- 



