ANODONTA 65 



ganglion to the auditory organ or otocyst, a sma/1 vesicle 

 lying a little way behind the ganglion and containing a 

 calcareous concretion or otolith. The cerebro-visceral con- 

 nectives run backwards on each side through the visceral 

 mass, traverse the lower surface of the glandular part of the 

 excretory organ, and end in a pair of closely apposed ganglia 

 -on the under surface of the posterior adductor muscle. These 

 visceral-ganglia are only covered by a layer of columnar 

 epithelial cells, supposed to represent a patch of sensory 

 epithelium very generally associated with the ctenidia of 

 molluscs, and known as the osphradium. 



The sexes in Anodonta are separate. The gonads in both 

 sexes are simple, consisting of a mass of ovarian or testicular 

 tubes ramifying in the visceral mass. The openings of the 

 gonads have been described ; they are just below the excretory 

 pores. "he ova are swept into the cloacal chamber by the 

 incurrent stream of water, are fertilised there, and afterwards 

 passed into the inter-lamellar spaces of the outer gill-plate, 

 where they undergo segmentation and go through the early 

 stages of development. Thousands of embryos may be found 

 in this position in the summer and autumn months, but they 

 do not develop further, and remain in the brood-pouch formed 

 by the inter-lamellar chamber till the following spring. They 

 then emerge as peculiar larval forms known as Glochidia which 

 are retained by the parent in the brood-pouch until some 

 fish passes in the neighbourhood, when they are at once 

 ejected. A Glochidium differs a great deal from an 

 ordinary molluscan larva. It has a shell composed of a 

 pair of triangular valves, hinged together along the base line. 

 The apex of each triangular valve is turned inwards and forms 

 a stout hook, the outer surface of which is covered with spines. 

 There is only a single adductor muscle at the anterior end of 

 the body, and immediately behind this is a gland which secretes 

 a long sticky thread called the "byssus. There is no foot, but 

 the mantle lobes are large and thick, and each is furnished with 

 four peculiar sensory organs in the form of tufts of bristles 

 seated on as many large epidermic cells. The mouth is re- 

 presented by a stomodaeal invagination, and there is a small 

 gut or enteron, but no anus. Traces of the nervous system 

 are present, and a pair of lateral pits may possibly represent 

 transitory sense organs. Posteriorly there is a tuft of cilia at 



II. E 



