Ml 



MOLLUSC A. 



MOLLUSC A. 



H.I 



complete sheath for, the head, are produced into eight or ten procoates, 

 UK so-called anna, on which are act the acetabula, or tuckers. 



Vertical Section of Lollga mrrlia. 



, oral aperture ; , anal aperture, or extremity of the Intmtine ; m, mantle; 

 m, >bell ; I, bnnchlB ; z, cerebral ganglia ; y, pedal ganglia j , paricto- 

 splanchnic ganglia ; rp t funnel. 



Beyond thii peculiar arrangement and development of the external 

 organ*, we are not aware that any character* exist by which the 

 Ctphalopoda, on a clam, can be distinguished from the other Mol/iuca. 

 Among themselves they present a remarkable harmony, differing 

 chiefly in the number of their branching in the internal or external 

 poaition of their shell, and in the nature of the appendages into which 

 the edges of the foot are modified characters which do not attain to 

 ordinal importance in other divisions of the Molliuca. 



Having thus glanced at all the leading modifications of the Neural 

 Plan, we may next turn to the Hmmal Plan, commencing with its 

 Mnltuacoid modification constituted by the Aicitlioida alone. 



The Aiddioida. A* a Molltucoid group, the Ascidians are charac- 

 terised, in the first place, by the rudimentary condition of their whole 

 neural region, and by the reduction of their nervous system to a 

 ingle infra-cosophageal ganglion. Betides these however, their organ- 

 isation present* certain characters which appear at first sight very 

 remote from such a Common Plan as has been described, and 

 hardly deducible from it An Ascidian, in fact, is usually fixed by 

 one extremity of iU body, and presents at the other two apertures. 

 One of these leads into a wide cavity, whose entrance is fringed with 

 a circlet of tentacles, and whose walls (except along the middle line 

 anteriorly and posteriorly) are perforated by innumerable ciliated 

 apertures, and often thrown into folds, by which their surface it 

 greatly increased. At the bottom of this cavity the branchial sac 

 a second wide aperture leads into the alimentary canal, which invari- 

 ably present* a ha>mal flexure, and then almost always bends back- 

 wards neurally to terminate in a second wide cavity. This, the 

 atrium, whose more external portion is usually termed the cloaca, 

 open* externally by the second or cloactl aperture, and extends along 

 each aide of the branchial sac tip to its median line of attachment 

 communicating freely with its cavity by means of the small ciliated 



apertures which have been mentioned. The tingle ganglion lies 

 between the oral and cloaca] aperture*. 



AtriJioula,\, Bollenia. 2, Oynlhia. 3, BotryUut. 4, Intestine of Ptnpkara. 

 5, Clarelina. 6, Salpa. 7, Apprndicularia. 



a, oral aperture; *, anal aperture, or tbe extremity of the InteMlne; rf, 

 cloacal aperture and atrium ; /, branchial sac ; f, hypo.phar}-ngeal band ; m', 

 tent ; y, genitalia ; y, pedal ganglia. 



Now, in what manner is thia form derivable from the Archetype ? 

 It is to be remarked, in the first place, that the pharynx, large in tin- 

 Polyzoa, becomes comparatively enormous in the Ascidians; whiK- 

 the tentacles, which were very large in the Polyzoa, are in the Ascidians 

 comparatively small. Next, with the development of a post-abdomen, 

 the intestine acquires a htemal flexure; but instead of the anal 

 aperture remaining on the hrcmal side, it is bent round, by the same 

 process as in Spirialit and Limacina, but in the inverse direction. 

 Suppose with all this that a mantle has been developed, and that iu 

 free margin remaining small and narrow, has followed the anus to tho 

 neural side, while its cavity has extended up on each side of the 

 pharynx to the middle line of the haemal surface of the latter, carry- 

 ing to a great extent a process of which the outline may be seen in 

 Cymbulia, and giving rise to the atrium ; imagine also that the aac 

 thus constituted externally by the inner surface of the mantle (third 

 tunic), and internally by the pharynx, becomes perforated by minute 

 apertures and the result would be an Ascidian. 



Such is the manner in which the Ascidian type is derivable from the 

 Common Plan. Of this type the group present* three subordinate nx >. I i 

 fi cation*. The first is that presented by the extraordinary and instructive 

 genus Appcndiculana (fg. ~, 7), which in a manner represents per- 

 manently the larval state of the more perfect members of the group- 

 swimming by means of a long rapidly-vibrating tail, like that of a 

 tadpole. In Appendicularia there is no cloacal aperture or atrium. 

 The mouth opens into a wide pharynx representing the branchial 

 sao of other Axcidians ; from this a gullet leads into the storanch. 

 The narrower intestine passes from the stomach, forwards and to the 

 lurtnal surface, where it terminates without bending downward*, and 

 without being surrounded by any special cavity. Appendicularia 

 therefore might be said to be a form in which the process of modifi- 

 cation of the HolluRcan Archetype into the Ascidian Type is arrestrd 

 half way. 



In all other Ascidians this process is complete, and there is a distinct. 

 cloacal aperture and atrium ; but these forms again may be arnmi.;.-.! 

 under two great sub-typical modification*, according to the develop- 

 ment of the branchial sac relatively to that of th<) post-abdomen. 

 In such form* as Cynthia, Bollenia, Perophora, Botrylltu, the branchial 

 sac attains so great a proportional size as to occupy the whole, or 

 nearly the whole, length of the body, the intestine lying on nnr si.],' 

 of it : these might therefore be well denominated Atci'lim Rranrli inln, 

 Branchial Aftcidians. On the other hand, in Clavdina, Aiiliilittm, 

 1'olyclmum, Satpa, the alimentary canal lies completely behind the 



