186 MR J. D. MACDONALD ON THE ZOOLOGICAL CHARACTERS 



with the four long, conical, and perfectly retractile arms, which spring imme- 

 diately in front of them. These arms are dotted over with minute rudimentary 

 suckers, arranged quiucuncially, and from between them arises a curious shovel- 

 shaped and retractile proboscis. This latter organ is broad and depressed, termi- 

 nating in a point anteriorly, and having, on the upper surface, a large oral opening 

 with a thin circular lip. The dental armature of this proboscis is, with the excep- 

 tion of that of Pneumodermon, the most formidable amongst moUusca. In the 

 middle of the floor of the mouth, and quite exposed from above, is a globular 

 tongue, mainly composed of two broadly oval cartilages, overlaid with a lingual 

 pavement of teeth, leading into a short saccule posteriorly. The median series of 

 plates are crescentic, with the concavity directed backwards, and armed with a 

 principal conical fang in the middle, and a rudimentary one on either side. The 

 lateral plates are numerous, and bear a simple conical tooth on the inner side, 

 with a small shoulder externally. 



In front of the tongue, the anterior or inferior lip is furnished with a transverse 

 row of minute hooks, one in the centre, and an outlying one on either side, being 

 larger than the rest. Behind the position of the tongue, and on each side of the 

 oral cavity, is a shallow evertile pouch, lined with large gently curved conical 

 teeth, which appear to enjoy a twisting or cork-screw action while they are being 

 everted, so as to pierce and secure living prey when both cheek-pouches are 

 brought forcibly in apposition. The upper or posterior lip is wholly unarmed, so 

 as to admit of its expansion in receiving prey thus seized, probably torn up and 

 forced backwards into the gullet by this most efficient and wonderful armature. 

 Speaking of the lateral oval plates of Pleiirohranchcca, Siebold observes, — " To 

 the same category belong the spines which Eschricht found upon the pharynx 

 of a Clio, and described as jaws placed laterally, as in the Articulata, and fur- 

 nished with long sharp comb-like projections or teeth," giving a very false idea of 

 the organs just noticed in Clio caudata, if they are indeed the same or similar in 

 the northern Clio. 



The description of the head, tentacula, arms, proboscis, and dentition of Clio 

 caudata just given, for the simple reason that it has been taken from life under 

 favourable circumstances, will be found to agree but little even with the recog- 

 nised characters of the genus, at least as they occur in systematic works. I have 

 observed, moreover, that without any other apparent distinguishing feature some 

 of the southern Clios had simple tapering tails {Clio a longue queue of the French), 

 while others have three prominent ridges (the dorsal one frilled) meeting in a 

 point posteriorly, so as to give a depressed trigonal section. 



On comparing a Clio with a thecasomatous pteropod, Hyaloea for example, 

 the mouth and dental system in the former case will be seen to occupy the most 

 advanced position, while in the latter they have receded so far within the limits 

 of the foot as to give the animal an acephalous, appearance, and this view 



