MOLLUSCA. 267 



treniity and bears numerous sharp spines on its outer side, which are 

 especially large in the median line (vide fig. 121 A). It is employed in fixing 

 the larva, after it is hatched, on to the fish on which it is for some time 

 parasitic. The shell is perforated by numerous pores. 



After the shell has become formed a new structure makes its appearance 

 which is known as the byssus-gland. It is developed as an invagination of 

 the epiblast at the hinder end of the body : Rabl was unable to determine 

 whether it was formed from the three large epiblastic cells present there or 

 no. It subsequently forms an elongated gland with three coils or so round 

 the adductor muscle on the left side of the body, but opening in the median 

 ventral line. It secretes an elongated cord by which the larva becomes 

 suspended after hatching. 



For some time the ventral portion of the body projects behind the ends 

 of the valves of the shell, but before these are completely formed a median 

 invagination of the body wall takes place, which obliterates to a large extent 

 the body cavity, and gives rise to two great lateral lobes, one for each valve. 

 These lobes are the mantle lobes. 



Before the mantle lobes are fully formed peculiar sense-organs, usually 

 four in number, make their appearance on each lobe. Each of them consists 

 of a columnar cell, bearing at its free end a cuticle from which numerous 

 fine bristles proceed. Covering the cell and the parts adjoining it is a 

 delicate membrane perforated for the passage of the bristles. The largest 

 and first formed of these organs is placed near the anterior and dorsal part 

 of the mantle. The three others are placed near the free end of the mantle 

 (vide fig. 121 A). These organs probably have the function of enabling the 

 larva to detect the passage of a fish in its vicinity, and to assist it therefore 

 in attaching itself. When the embryo is nearly ripe there appears im- 

 mediately ventral to and behind the velum a shallow pit on each side of the 

 middle line, and the two pits appear to be connected by a median transverse 

 bridge. These structures have been the cause of great perplexity to different 

 investigators, and their meaning is not yet clear. According to Rabl the 

 median structure is the somewhat bilobed archenteron, and according to 

 his view it is not really connected with the laterally placed pits. The cilia 

 of the velum overlie these latter structures and make them appear as if their 

 edges were ciliated. They are regarded by Rabl as the rudiments of the 

 nervous system. 



With the development of the shell, the mantle, and the sense-organs, the 

 young mussel reaches its full larval development, and is now known as a 

 Glochidium (fig. 121 A). 



If the parent, with Glochidia in its gills, is placed in a tank with fish, it 

 very soon (as I have found from numerous experiments) ejects the larvae 

 from its gills, and as soon as this occurs the larvae become free from the egg- 

 membrane, attach themselves by the byssus-cord, and when suspended in 

 this position continually close and open their shells by the contraction of the 

 adductor muscle. If the mussels are not placed in a tank with fish the larvae 

 may remain for a long time in the gills. 



