CIRRHOPODA. 



355 



perforated by a minute aperture ; but the real nature of this instru- 

 ment we shall examine by and by. 



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

 struction of Pentalasmis, 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 as a Mollusk would 

 seem to be at once demonstrable, for, in fact, most conchologists 

 agree in claiming these animals as belonging to their own de- 

 partment ; 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 (Jig. 167, &, b) to Tepresent the 

 thoracic portion of a Crustacean slightly bent upon itself, and 

 enclosed in an extensively developed thorax ;* the valves of the 

 shell would represent this thorax, which would be divided into five 

 pieces ; the first pair of cirrhi arising from the body would then 

 represent the true feet of a Crustacean ; the branchiae would 

 occupy the same position in both ; the rest of the body of the 

 Barnacle, namely, that which supports the five other pairs of feet, 

 would represent the tail of the Crustacean, and the ciliated, nata- 

 tory feet, generally connected with that part of the external skele- 

 ton : even the mouth, as the author referred to might have 

 added, with its triple series of jaws, is more nearly allied in 

 structure to that of the Crustaceans Ffc.168. 



than to anything we shall meet with 

 in the structure of the oral organs of 

 true Mollusca. 



(391.) But the affinity which unites 

 the Cirrhopoda to the Homogangliata 

 is not merely exemplified in the analo- 

 gies that can be pointed out between 

 the external configuration of Pentalas- 

 mis and some Crustacean forms ; the 

 nervous system even, as we might be 

 led to anticipate from the symmetrical 

 arrangement of the articulated cirrhi, 

 still exhibits the Homogangliate con- 

 dition, and, besides the supra-cesopha- 

 geal masses, forms a longitudinal chain 

 of double ganglia arranged along the 



* Cuvier, MSmoire sur les Animaux des Anatifes et des Balanes, et sur leur 

 Anatomic, p. 6. 



2 A2 



