450 



CIREHOPODA. 



Fig. 232. 



diately behind the mouth, we find on each side certain pyramidal fleshy 

 appendages (d d d), resembling, as Hunter expressed it, a minute Star- 

 fish, which no doubt constitute the branchial or respiratory organs. 

 Commencing above the mouth, we further notice on each side six pairs 

 of articulated and flexible 

 arms or cirri (fig. 232, c c), 

 each being composed of a 

 series of semi - corneous 

 pieces, and exhibiting at 

 each joint long and stiff 

 hairs. Every pair of cirri 

 arises from a single promi- 

 nent stem ; and those most 

 distant from the mouth being 

 the longest and most exten- 

 sile, the whole apparatus, 

 consisting of twenty -four 

 cirri, forms, when protruded 

 from the body, a kind of net 

 of exquisite contrivance, in 

 which passing particles of 

 nourishment are easily en- 

 tangled, and thus conveyed 

 to the mouth. Lastly, on 

 separating the cirriferous 

 pedicles, we find, termina- 

 ting the body, and forming, 

 as it were, a kind of tail, a 

 long, soft, and flexible organ 

 (fig. 235, fc), the extremity 

 of which is perforated by a 

 minute aperture ; but the 

 real nature of this instrument we shall examine by and by. 



(1165.) On reviewing this general description of the external con- 

 struction of Pentelasmis, the reader cannot but be struck with the 

 singular combination of characters which it exhibits. Judging from its 

 shell alone, its right to be considered a Mollusk would seem to be at 

 once demonstrable ; for, in fact, most conchologists agree in claiming 

 these animals as belonging to their own department ; and yet if, after 

 removing the shell, we compare the animal with a Crustacean, its 

 alliance with that class is equally evident. Suppose the body (fig. 232, 

 6 b) to represent the thoracic portion of a Crustacean slightly bent upon 

 itself, and enclosed in an extensively developed thorax* ; the valves of 



* Cuvier, M6moire BUT les Animaux des Anatifes et des Balanes, et sur leur 

 Anatomie, p. 6. 



Pentelastiiia vitrea : a a, the shelly valves ; b b, body 

 contained within the shells ; c c, the cirri ; d d d, pre- 

 sumed branchial apparatus ; e,f, muscular expansions; 

 g, the mouth. (After Hunter.) 



