THE TRANSITION TO THE WHI/r. 57 



A cyst Boon forms From the tissues of the fish and encloses the 

 parasitic Glochidium. A peculiar mushroom-like growth formed by 

 large cylindrical rolls of the embryonic mantle serves for absorbing 



the tissues (if the host, and especially the tin-rays in which the shell- 

 hooks are embedded. The larva is no doubt nourished in this way 

 until its intestinal canal becomes functional. 



'The time during which the Glochidium remains parasitic on the 

 fish appears to be determined by the favourable or unfavourable 

 conditions of temperature, and varies from a few weeks to several 

 months. Schierholz and Beaun found that the larvae remained 

 seventy-two to seventy-three days on the fish, during which time 

 they develop the definitive form. 



The South American relations of our Anodonta have larvae differing greatly 

 in shape, so that v. Jhering, who found these larvae within the mantle-cavity 

 of the parent, would have taken them for parasites had not all doubt as to 

 their being Lamellibranchs been removed by the agreement of the egg- 

 envelope and its micropyle with the envelope of the ovarian eggs (No. 25). In 

 these forms the embryos are found in the inner gills, not, as in our native 

 Uwionidae, in the outer gills. The body in the South American forms is com- 

 posed of three sections : (1) a conical anterior portion covered with cilia ; 

 (2) a large middle portion containing internally the entoderm-elements and 

 two kidney-like structures (byssal glands ?) ; the dorsal side of this region is 

 only partly covered by a delicate shell-integument; (3) the short caudal end. 

 which forks, and consequently terminates in two rounded prominences, beset 

 with bent hook-like setae. 



A very peculiar organ possessed by these larvae is a very thin but broad and 

 rlat band considered by v. Jhering to be the byssus. This band is almost at 

 the middle of the body and is attached to the ventral surface, from which it 

 runs forward. It is somewhat broader than the body, and six to ten times as 

 long. It is said also to be connected with the anterior part of the body. 



According to the somewhat vague account given by v. Jhering of the larva 

 named by him "Lasidium," and in the absence of any statements as to the 

 development of this larval form, it is at present impossible to compare it with 

 the entirely different Unionid larvae (Glochidia) or with the larvae of other 

 Lamellibranchs. 



C. The Transition to the Adult. 



Very soon after attachment, as early as the second day. the larval 

 organs which enabled the Glochidium to establish itself on its host, viz., 

 the glutinous filament and the brush-like sensory organs, degenerate. 

 A wide pit like depression of the ventral surface arises behind these 

 organs as they degenerate ; this depression involves the two lateral 

 pits already present in the embryo (Fig 2<> A and B } g). At this 



