June 1, I860.] 



HARDWICKE'S SCIENCE-GOSSIP. 



123 



THE ANATOMY OF THE ZEBRA MUSSEL 



(Dreissena polymorphic) . 



"TVREISSENA POLYMORPHA is originally an 

 •^-^ inhabitant of the rivers in the Aralo-Caspian 

 province. 



Thence it found its way into the European 

 rivers, and was recognized by Linnaeus as a member 

 of the European fauna. 



The shell in the early part of this century became 

 well known to collectors under the names of 

 Mytilus polymorphs and Mytilus volgensis ; but it 

 did not receive much attention from naturalists, 

 and, in fact, its true zoological characters were 

 entirely unknown until the year 1826. 



At this date it was found by M. Dreissens, an 

 apothecary of Louvain, in a river near that city, 

 and was by him sent to M. Van Beneden, director 

 of the Louvain Museum, who described its anatomy 

 minutely before the Erench Academy, pointed out 

 in what manner it differed from Mytilus, with which 

 genus it had been placed, and, recognizing it as the 

 type of a new genus, named it Dreissena, after 

 M. Dreissens. Van Beneden' s paper on the subject 

 is published in the Annates de Sciences Naturelles 

 for 1826. 



-a 



Fig. 88. Animal and shell of Dreissena. a, a, siphons. 



Its first discoverer in England was Mr. J. 

 Sowerby, who found it in 1824 at the Surrey 

 Docks, which it is supposed to have reached on 

 some floating timber. It was next found on timber 

 at Wisbeaeh in 1828 ; then in the Union Canal, 

 Scotland, in 1S31 ; and in the Neone in 1836 ; 

 finally, at Exmouth, in 1816. We have thus five 

 distinct centres of introduction into Great Britain. 

 Erom these Dreissena has spread far and wide, and 

 is to be found throughout the vast canal system in 

 England and in the several rivers connected with 

 it. As far as I have been able to ascertain, it is 

 not found in Ireland. 



Let us now turn to the anatomy of the Dreissena. 



1. Of the nervous system. — This consists, as in 

 most of the acephalous mollusks, of several pairs of 

 ganglia. 



One pair lies immediately over the oesophagus, 

 and sends out various branching nerves to the labial 

 appendages— the mouth and the parts immediately 

 surrounding it. Another and larger pair, called the 

 branchial ganglion, is situated near the posterior 

 adductor muscle, and supplies the branchiae and 

 surrounding parts with nerves. A single small 

 ganglia, with its nerve branches, is placed near the 

 siphons. 



Such is the very simple nervous system of 

 Dreissena. The various senses must of necessity 

 be but slightly developed. Eyes, although present 

 in some bivalves in a rudimentary state, do not 

 appear to exist in the Dreissena : the senses of smell 

 and taste, if they exist at all, must be of the 

 weakest. The auditory capsule, which has been 

 detected in some few bivalves, does not appear to 

 be present in this one : the sense of touch, however, 

 is evidently rather delicate. 



a 



Fig-. S9. Circulatory system, a, a, branchiae ; b, the heart.* 



2. The circulatory system. — The heart, which is 

 situated on the back of the animal immediately 

 under the skin, consists of a single central ventricle 

 and two auricules, one on each side. These receive 

 the blood from the branchiae, of which there are 

 two pairs, one on each side of the animal, com- 

 municating freely with the branchial cavity. The 

 auricles in] turn transmit the blood to the ventricle, 

 whence it is distributed through the body. 



3. The respiratory system. — Respiration in the 

 Dreissena, as in other bivalves, consists in the ex- 

 posure of the blood to the influence of water con- 

 taining air. This process is performed by means 

 of the movements of the mantle, with its siphons, 

 and the gills, or branchiae. As before mentioned, 

 there is a pair of gills on each side of the body. 

 Each gill is composed of two laminae, divided 



* The ■woodcuts are engraved fac-simile from the author's 

 sketches. 



G 2 



