121 



HATiDWICKE'S SCIENCE-GOSSIP. 



[June 1, 1869. 



internally by septa into a series of narrow chambers, 

 the divisions being marked externally by lines. 

 These chambers open into longitudinal channels at 

 the base of the gills, which unite at the commence- 

 ment of the exhalant siphon. Under the micro- 

 scope the gill lamina3 exhibit a network of blood- 

 vessels, fringed with cilia, opening into the gill- 

 tubes. 



The mantle which envelops the whole animal 

 has three openings — one a small slit on the side, 

 through which the foot and byssus pass ; the others 

 are in front, and are prolonged into siphons. One, 

 the branchiala inhalent, has its orifice surrounded 

 by a double fringe of tentacles ; through this the 

 water enters, distributes itself over the gills, and 

 passes out at the other, or exhalent siphon ; thus a 

 continual current of water flowing in and out is 

 constantly kept up. 



Fig. 90. Digestive system, a, a, labial tentacles ; b, stomach; 

 c, caecal appendage ; d, intestine. 



4. The digestive system.— The mouth is placed 

 between the anterior nervous ganglia. The orifice 

 is protected by two lips, which are prolonged on 

 eaca side into a pair of membranous palpi: these 

 are called the labial tentacles. The oesophagus is 

 short, of the same size as the mouth, and is entirely 

 enveloped by the ovary and liver. The stomach is 

 situated between the heart and the anterior 

 adductor muscle, and is also enveloped on all sides 

 by the ovary and liver. This viscus is irregularly 

 round in form ; the upper surface is smooth and 

 homogeneous, the lower is permeated by numerous 

 small openings which give entrance to the biliary 

 ducts, On the left side there is a long csecal 

 appendage nearly equal in length to the intestine, 

 but of finer texture. 



The intestine which springs directly from the 

 lower end of the stomach is thick and whitish. It 



runs obliquely forwards, from left to right of the 

 median line of the body, plunges into the liver at 

 its posterior extremity ; there it bends round, and 

 returns upon itself on gaining the back of the 

 animal where it is immediately under the skin ; 

 then, after passing round the heart, it again beuds 

 upon itself just above the posterior adductor 

 muscle, and terminates at the exhalent orifice of the 

 mantle. 



The liver is large, forming with the ovary, with 

 which it is ultimately united, although not of 

 course anatomically connected, by far the largest 

 portion of the body. It pours its secretion by a 

 number of minute canals into the under surface of 

 the stomach, as before mentioned. Its chief mass 

 is situated over the stomach, and it extends forwards 

 and backwards. 



5. The reproductive system. — As before mentioned, 

 the ovary is of great size. It is at once observable 

 on removing the shell, from its brilliant yellow 

 colour. 



-a 



Fig. 91. Animal with left valve, and a portion of the mantle 

 removed («), showing b, the ovary; c, gland of Bojanus. 



Its surface has an areolar appearance, caused 

 by the fine granules of which it is composed. The 

 Dreissena is monoecious. 



In connection with the reproductive system, it is 

 necessary to mention the existence in Dreissena of 

 that peculiar viscus called the gland of Bojanus, 

 which in this animal is a small, dark-coloured body, 

 situated at the apex of the shell. The structure of 

 this gland is highly complex, and it has long been 

 a puzzle to zoologists. It is composed of several 

 cavities opening into each other. Prom its position 

 and other circumstances, it is probable that it plays 

 some part in the process of reproduction, but what 

 is at present unknown. 



0. The muscular system. — The muscular system is 

 triple, or, rather, consists of three distinct muscles, 

 two of which serve to move the valves of the shell, 

 and a third which regulates the movements of the 

 mantle with its siphons, and the foot and byssus. 

 The former, named the adductors, are placed at the 

 extremities of the valves of the shell, — one situated 

 on a small shelf at the apex called the anterior, and 



