MOLLUSC A. 221 



wards till it meets the epiblast anteriorly, and at the same time detaches 

 itself from the epiblast in the region where the blastopore was placed. 

 Where it comes in contact with the wall of the body in front a small 

 epiblastic invagination arises, which meets and opens into the arch enteric 

 sack and forms the permanent mouth. 



While these changes have- been taking place the shell is formed as a 

 continuous saddle-shaped plate on the dorsal surface. From this plate the 

 two valves are subsequently differentiated. On the dorsal surface they 

 meet with a straight hinge-line. Each valve is at first rounded, but 

 subsequently becomes triangular with the hinge-line as base. The valves 

 are not quite equi-sided, but the anterior side is less convex than the 

 posterior. At a later period a beak-shaped organ is formed at the apex 

 of each valve in the same manner as the remainder of the shell. This 

 organ is placed at about a right angle with the main portion of the valve. 

 It is pointed at its extremity 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 con- 

 sists 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 immediately 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 

 bv Rabl as the rudiments of the nervous system. 



