1 56 DE. H. LYSTER JAMESON ON THE [Mar. 4, 



which I found the Sporocysts in Cardium at Piel corresponds to 

 the end of the anterior pallial artery. 



The Cercaria in the Sporocyst first appears as a little oval 

 cellular ball, budded off from the wall, measuring about '05 mm. 

 During the early stages of its development it is transparent, and 

 its structure can be made out without difficulty ; but, as it grows, 

 its excretory organs become gradually laden and distended with 

 opaque granules, which conceal the other parts. 



The fully formed Oercariae in the Sporocysts measure 'IS mm, 

 to "3 mm. They are whiter than those found in Mytilus, but the 

 arrangement of the spines on the cuticle is the same. They 

 possess a pair of pyramidal or conical light brown eye-spots 

 (PI. XVI. fig. 10, e.), each provided with a lens. There are about 

 six tactile papillae {t.p.) at the anterior end of the body. These 

 sense-organs are no doubt serviceable to the larva, during its free 

 living stage, after leaving Tapes, and before entering Mytilus. 

 The digestive system (PI. XYI. figs. 9, 10, dig.) is empty, and 

 occupies less space than in the Mytilus stage. The form and 

 relations of the suckers and pharynx are the same. The penis 

 (PI. XYI. fig. 9, pe.) and testes are already developed, and have 

 the same relations as in the Mytilus worm. In pressure prepara- 

 tions of the live worm they are not easy to discern. 



The majority of Oercarise in the Sporocysts are fully developed, 

 young transparent ones being less common. They probably 

 remain a considerable time before vacating it. A few on their 

 way out may often be found free in the tissues of the mantle. 



Search in the mud and with the tow-net at Billiers, failed to 

 reveal the free living stage. I have, however, found examples in 

 water in which Tapes had been kept for some days. 



This Trematode is not provided with a cercarian tail at any 

 stage of its existence, and it is only capable of creeping movements. 

 The larva in the Sporocyst is rather more active than the later 

 stage which occurs in Mytilus. 



If a Cercaria dies while still in the Sporocyst, its remains 

 become calcified; bvit, not being enclosed in an epidermal sac, 

 secreted by the moUusc, it does not give rise to a pearl, but merely 

 to a concretion. Again, an exhausted Sporocyst may undergo 

 similar calcareous degeneration with the same result. 



Artificial Infection of Mytilus. 



On leaving Billiers in the beginning of September 1901, I 

 brought with me about fifty infected examples of Tapes. I first 

 placed these in a tank at the Piel Fish- Hatchery, which Professor 

 Herdman and Mr. Scott kindly placed at my disposal. In order 

 to test experimentally the infection of Mytilus from Tapes, I put 

 in the same tank about seventy mussels, taken from the piles of 

 the old pier at Piel. These mussels of which I examined a 

 number, were practically without parasites. About one in every 

 five of the largest examples contained a Cercaria, one had two 



