MOLLUSCA 



121 



With regard to internal organization wo may commence 

 with the disposition of the renal organ (nephridinm), the 

 external opening of which has already been noted. The 

 position of this opening and other features of the venal 

 organ have. l>een determined recently by Mr. J. T. Cunning- 

 ham, Fellow of University College, Oxford, who writes as 

 follows from Naples, February 1883 : 



"There is considerable uncertainty with respect to the names of 

 tlie species of Aplysia. There are two forms \vhirh are very common 

 in the Gulf of Naples, and which I have used in studying the ana- 

 fumy cil' tin 1 renal organ in the genus. One is <|uite Mark in colour, 

 and measures when outstretched eight or nine inches in length. 

 The other is light brown and somewhat smaller, its length usually 

 not exceeding seven inches. The tirst is flaccid and sluggish in its 

 movements, and lias not much power of contraction ; its epipoilial 

 lobes are enormously developed and extend far forward along the 

 hinlv ; it gives out when handled an abundance of purple liquid, 

 which is derived from cutaneous glands situated on the under side 

 of the free edge of the mantle. In the Zoological Station this form 

 is known as AJI. It'fiorltia ; but according to Blochmann it is iden- 

 tical with A. Camel us of Cuvier. The other species is -/. </ /ii/nnx ; 

 it is firm to the touch, and contracts forcibly when irritated ; the 

 secretion of the mantle-glands is not abundant, and is milky white 

 in appearance. The kidney has similar relations in both genera, 

 and is identical with the organ spoken of by many authors as the 

 triangular gland. Its superficial extent is seen when the folds 

 covering the shell are cut away and the shell removed ; the external 

 surface forms a triangle with its base bordering the pericardium and 

 its apex directed posteriorly and reaching to the left-hand posterior 

 corner of the shell-chamber. The dorsal surface of the kidney 

 extends to the left beyond the shell-chamber beneath the skin in 

 tin 1 space between the shell-chamber and the left epipodium. 



" When the animal is turned on its left-hand side ami the mantle- 

 chamber widely opened, the gill being turned over to the left, a 

 part of the kidney is seen beneath the skin between the attachment 

 of the gill ami the right epipodium (fig. 63). On examination 

 this is found to be the under surface of the posterior limb of the 

 gland, the upper surface of which has just been described as lying 

 beneath the shell. In the posterior third of this portion, close to 

 that edge which is adjacent to the base of the gill, is the external 

 opening (tig. 63, o). 



" When the pericardium is cut open from above in an animal 

 otherwise entire, the anterior face of the kidney is seen forming 

 the posterior wall of the pericardial chamber ; on the deep edge of 

 this face, a little to the left of the attachment of the auricle to the 

 floor of the pericardium, is seen a depression ; this depression con- 

 tains the opening from the pericardium into the kidney. 



"To complete the account of the relations of the organ : the right 

 anterior corner can be. seen superficially in the wall of the mantle- 

 chamber above the gill. Thus the base of the gill passes in a slant- 

 ing direction across the right-hand side of the kidney, the posterior 

 end being dorsal to the apex of the gland, and the anterior end 

 ventral to the right-hand corner. 



"As so great a part of the whole surface of the kidney lies adjacent 

 to external surfaces of the body, the remaining part which faces 

 the internal organs is small ; it consists of the left part of the under 

 surface ; it is level with the floor of the pericardium, and lies over 

 the globular mass formed by the liver and convoluted intestine. 



"Mere dissection does not give sufficient evidence concerning such 

 communications as these of the kidney in Aplysia. I studied the 

 external opening by taking a series of sections through the sur- 

 rounding region of the gland ; to demonstrate the internal aperture 

 injected a solution of Berlin blue into the pericardium ; it did not 

 fill the whole kidney easily, but ran down iuto the part adjacent to 

 the base of the gill." 



Thus the renal organ of Aplysia is shown to conform to 

 the Molluscan type. The heart lying within the adjacent 

 pericardium has the usual form, a single auricle and ven- 

 tricle. The vascular system is not extensive, the arteries 

 soon ending in the well-marked spongy tissue which builds 

 up the muscular foot, epipodia, and dorsal body-wall. 



The alimentary canal commences with the usual buccal 

 mass ; the lips are cartilaginous, but not armed with horny 

 .jaws, though these are common in other Opisthobranchs ; 

 the lingual ribbon is rnultidenticulate, and a pair of salivary 

 glands pour in their secretion. The oesophagus expands 

 into a curious gizzard, which is armed internally with large 

 horny processes, some broad and thick, others spinous, fitted 

 to act as crushing instruments. From this we pass to a 

 stomach and a coil of intestine embedded in the lobes of a 

 voluminous liver ; a caecum of large size is given off near 



Ihi' cotiuiH mi MM nt .if tin- inlestlne. The livrr opens by 

 two ducts into flu- digestive tract. 



The generative organs lie close to the foil of intestine 

 and liver, a little to the leftside. When dissected out" they 

 appear as represented in fig. 04. The essential reproductive, 



FIG. 65. Follicles of the hermaphrodite monads of Euthynenrous Anisopleura. 

 A t of Helix ; P>, of Eolidia. o, ova ; ft, developing spennatozoids ; -, com- 

 mon efferent duct. 



organ or gonad consists of both ovarian and testicular 

 cells (see fig. 65). It is an ovo-testis. From it passes a 

 common or hermaphrodite duct, which very soon becomes 

 entwined in the spire of a gland the albuminiparous gland. 

 The latter opens into the common duct at the point x, and 

 here also is a small diverticulum of the duct y. Passing 

 on, we find not far from the genital pore a glandular spherical 

 body (the spermatheca a) opening by means of a longish 

 duct into the common duct, and 

 then we reach the pore (fig. 6.3, 

 I'). Here the female apparatus 

 terminates. But when the male 

 secretion of the ovo-testis is 

 active, the seminal fluid passes 

 from the genital pore along the 

 spermatic groove (fig. 63,) to 

 the penis, and is by the aid of 

 that eversible muscular organ 

 introduced into the genital pore 

 of a second Aplysia, whence it 

 passes into the spermatheca, there 

 to await the activity of the fe- 

 male element of the ovo-testis of 

 this second Aplysia. After an 

 interval of some days possibly 

 weeks the ova of the second 

 Aplysia commence to descend 

 the hermaphrodite duct ; they 

 become enclosed in a viscid secre- FIG. 66. 

 tion at the point where the al- { 

 buminiparous gland opens into S, all of which are not figured ; 

 ,, , * . ' . , * . , ., e, hind gut; on, anus. (From 



the duct intertwined With it ; Gegenliaur, after Alder and Han- 



and on reaching the point where COL ' k -) 

 the spermathecal duct debouches they are impregnated by 

 the spermatozoa which escape now from the spermatheca 

 and meet the ova. 



The development of Aplysia from the egg presents many 

 points of interest from the point of view of comparative 

 embryology, but in relation to the morphology of the 

 Opisthobranchia it is sufficient to point to the occurrence 

 of a trochosphere and a veliger stage (fig. 60), and of a 

 shell-gland or primitive shell-sac (fig. OS, /*.<), which is suc- 

 ceeded by a nautiloid shell. 



The nervous system of Aplysia will be found on com- 

 parison of fig. 20, which represents it, with our schematic 

 Mollusc (fig. 1, D) to present but little modification. It is 

 in fact a nervous system in which the great ganglion-pairs 

 are well developed and distinct. The Euthyneurous visceral 

 loop is long, and presents only one ganglion (in Aplysia 

 but two distinct ganglia joined to one another in 



