104 DIGESTIVE ORGANS. 



always a distinct nucleus and its contents consist sometimes of 

 a yellowish or greenish fluid, and within are concentrically lami- 

 nated urinary concretions. By bursting, these secretory cells 

 allow the urinary concretions to escape into the water in the 

 kidney. 



Externally, the kidney is spun over by a very thick and strong 

 vascular network of the venous system. This vascular network 

 has several openings into the kidney, through which the urinary 

 products are mingled with the blood ; and, consequently, when 

 these are examined microscopically, they are found to contain, 

 besides renal cells and free concretions, a considerable number 

 of blood-corpuscles. 



Usually the kidney opens into the base of the respiratory 

 cavity by means of a transverse, slit-like opening, encircled by a 

 strong sphincter muscle ; Triton, Dolium, Cassis, Murex,Littorina, 

 Natica, etc.) ; sometimes, however, it has a gut-like, efferent duct, 

 a ureter, which passes forward between the rectum and sexual 

 canal, opening outwards not far behind the anus (Paludina, 

 Turbo, Voluta, ConusX There are accordingly three efferent 

 canals alongside of each other at the right side of the respiratory 

 cavity, viz., the rectum, ureter, vagina or vas deferens. The 

 ureter is usually a somewhat dilated canal and is not simply a 

 prolongation of the kidney, but is often separated from the latter 

 by a diaphragm perforated by a number of holes which are 

 encircled by muscular fibres. Within, a number of longitudinal 

 folds are to be observed, and it is covered throughout with ciliated 

 epithelium. The ureter is usually filled with water and it is 

 possible that it may have some other significance than a mere 

 efferent duct. 



Pteropoda. This little group of pelagic animals are much 

 simpler in organization than the gastropods, and to a certain 

 extent may be comparable to the larval stage of some of the 

 latter. Their entire life being passed in the open sea, far from 

 any shelter save that of floating weed, they are active in habit, 

 swimming by the vigorous flapping of their natatory lobes, and, 

 as might be expected, small and fragile as they are, are carniv- 

 orous. Clio and Pneumodermon constitute the section Gymnoso- 

 mata, in which the head is distinct and the animal is without 

 shell ; these are provided with tentacles, the surface garnished 

 with suckers, and well adapted for seizing their microscopic prey. 

 There is also a lingual ribbon, well studded with teeth, and two 

 jaws, each consisting of an agglomeration of corneous spines. 

 But the Pneumodermon possesses an additional and very curious 

 prehensile organ which recalls the tentacular arms of the decapod 

 Cephalopoda ; in two long caeca placed at the posterior part of 

 the pharynx are lodged cylinders covered with recurved spinules, 

 and which like the proboscis of certain gastropods, the animal 



