PTEROPODA. 



173 



run transversely from the dorsal to the ventral 

 aspect of the body bounding and separating 

 their orifices. 



Locomotive Apparatus. The locomotive ap- 

 paratus of the Pteropoda is constructed upon 

 most peculiar principles, consisting of a pair 

 of fin-like expansions attached to each side of 

 the neck of the animal. 



These fins, or, as they are commonly called, 

 wings in the Clio have a very remarkable struc- 

 ture, the two being continuous with each other, 

 through the intervention of a central part, 

 which extends transversely across the neck of 

 the animal, so that the lateral expansions are 



only the free extremities of the same organ, 

 the whole apparatus representing, with curious 

 exactness, the double paddle used by the 

 Greenlanders in navigating their light double- 

 pointed canoes (Kajaks). The entire appa- 

 ratus is muscular, and consists of two layers, 

 precisely similar in their structure, which, at 

 their margins, overlie each other, but are 

 only connected together by means of cellular 

 tissue. The course of the muscular fibres 

 is shown in the annexed figure, representing 

 the whole of the swimming apparatus removed 

 from the body; in which the following parts 

 may be distinguished : , the anterior or 



Fig. 109. 



Clio Borealis. 

 Swimming apparatus detached. (.After Eschricht.) 



dorsal margin ; b t the posterior or ventral ex- 

 cavation ; c c y the posterior, transparent, tri- 

 angular lappets which bound the fin ; d d, the 

 posterior outer border ; e e, the posterior inner 

 border ; o, the central portion which traverses 

 the neck ; m m m m, commencement of the free 

 portions of the fins. 



In the cellular membrane interposed be- 

 tween the two muscular layers of the fin ap- 

 paratus, four or five large nerves are seen to 

 run a tortuous course, and to divide into in- 

 numerable fibrillae. Eschricht likewise ob- 

 served a considerable blood-vessel derived 

 from the ventricle of the heart (not from the 

 auricle"), mounting up and dividing to supply 

 each fin. 



Respiration and Circulation. According to 

 Cuvier's views the fins of the Pteropoda have 

 been very generally regarded as performing 

 likewise the functions of branchiae. " Their 

 surfaces, seen with the microscope, present a 

 net-work of vessels, so regular, so close, and 

 so delicate that it is impossible to doubt their 

 office : their connection with the internal ves- 

 sels and the heart, moreover, confirms this 

 idea." * Cuvier's opportunities of investi- 

 gating this point of their anatomy were, how- 

 ever, very limited ; a single specimen only, and 

 that long kept in spirits of wine, having been 

 at his disposal. Eschricht's researches do not 

 at all confirm this view of their nature; and it 

 appears clear that Cuvier mistook the net- 

 work of muscular fibres represented in the 

 preceding figure for vascular ramifications. 



* Cuvier, Mem. sur le Clio. 



The vessel likewise called by Cuvier " the 

 branchial vein," and which he regarded as re- 

 turning the blood from the branchiae to the 

 auricle of the heart, Eschricht assures us, does 

 not communicate with the auricle, but is de- 

 rived from the apex of the ventricle so as to 

 be evidently arterial, and not venous, in its 

 nature. 



With regard to the connexion which exists 

 between the fin-apparatus and the body of the 

 Clio, it would appear that its central muscular 

 basis passes directly through the neck, and is 

 only attached to the surrounding parts by 

 nerves, vessels, skin, and cellular membrane. 



Nervous system. The nerves of the Clio are 

 very easily traced, seeing that they are not 

 only of considerable size, but are likewise con- 

 spicuous, on account of their pale red colour, 

 at least while the specimens are tolerably 

 fresh. 



The oesophageal ring lies in the neck above 

 the centre of the fin-apparatus, and lodged 

 in its dorsal excavation. It is composed of 

 eight large and two small ganglia. Each gang- 

 lion is surrounded by a transparent invest- 

 ment, and is very evidently composed partly 

 of a reddish and partly of a white nervous 

 substance. Of the eight larger ganglia of the 

 circum-cesophageal ring the two anterior (fig. 

 113. 30, 1) are situated close together, upon 

 the dorsal aspect of the oesophagus ; the two 

 posterior (fig. 113.30, 4>) are likewise close 

 together, but beneath the cesophageal tube. 

 Of the four intermediate ganglia, two are situ- 

 ated close together on each side of the oesoph- 

 agus (fig. 1 13. 30, 2, 3), so that when viewed 



