136 



DE. T. DAVIDSON ON EECENT BEACHIOPODA. 



Fiff. 13. 



tlie number of the brancliise amounts to ton (woodcut, fig. 13, b). In the interior of 



the swollen ring there exists a cutaneous layer which seems to support all the hrauchia 



apparatus, and is pierced by a hole opposite each 



of the branchiiie. The branchiae, which are only 



prolongations of the swollen ring, are covered with 



Tibratile cilia, are provided with flexiljle muscles, 



and become erected through their owu elasticity. 



The extremities of the branchiae converge towards 



the opening of the mouth. 



In the following stage twelve tentacles are present, 

 and at the same time the shell is formed ; but no 

 tubular perforations are yet seen. Of the following 

 stases Kowalevsky has been able to trace but oue, 

 that of the gradual foruiatiou of the shell. 



In 187G Signor Seguenza described a small Cistella 

 under the name Aryiope bipUcata. Dr. Gwyn Jeffreys 

 considered it to be an abnormal form of C. cuneata, but 

 I would be more inclined to refer it to A. ncapoUtana ; 

 only one example was collected. 



In his valuable paper on the structure and de- 

 velopment of Argiope or Cistella neapolllana and C. 

 cuneata, Mr. A. E. Shipley* observes that " The body 

 of Argiope lies almost entirely in the dorsal shell, 

 whose internal skeleton consists of three triangular 

 plates : one median, the other two lateral and paired." 

 I find myself, in the specimens of those species, that there is present in the dorsal 

 valve one prominent submarginal septum only, the loop being two-lobed, adhering to 

 the septum, and more or less confluent \\\i\\ the valve. The shell is sometimes, when 

 fresh, sli"litly subtransparent, showing the outline of the lophophore and the red colour 

 of the egf^s and larvaj. Mr. Shipley describes with much care and detail the minute 

 structure of the shell; he says (p. 496 et seq.) that it " consists of a number of very- 

 fine calcareous spicules supported by an organic network On the outer side of 



the shell is a layer, in thickness about one eighth of the whole shell, where the 

 proportion of calcareous matter to organic is enormously increased. The spicules 



are here nearly square, and packed against one another like bricks Outside 



this is a third layer or periostracum. This is a thick structureless cuticle ; it completely 



covers the whole shell, and in the fresh state adheres very closely to it The shell 



is pierced by very numerous canals which run completely through the calcareous part, 

 but their outer end is covered by the cuticle. They are of uniform diameter in the first 

 part of their course through the first layer, but when they reach the second layer of King, 



thev expand and end in funnel shaped mouths They are, as a rule, single but 



in the posterior part of the perforate shell, Avhere it is unusually thick, I have seen 



* Mitthcilungeu a. d. Zool. Station zu Neapol, Bd. iv. p. 494. 1SS3. 



Larva of Ari/iope nenpolitana (after 

 Xowalevsky). 



Condition previous to the formation of the 

 shell. 6, branchial' ; /, muscular bundle 

 of the branchiie of the inner side ; Z, head 

 of oesophagus ; z, divaricator muscles ; i, 

 stomach ; in.p^ ventral muscles of the 

 peduncle ; mi, muscles going from the 

 ventral valve to the upper part of the 

 dorsal valve ; el, internal envelope ; p, 

 peduncle. 



