240 THE LIVER AND HYALINE STYLET chap. 



into a very large coecum (see Fig. 144) or expansion of the 

 same, which serves as a reservoir for the biliary secretions. At 

 the point of connexion between the coecum and stomach is found 

 a valve, which opens for the issue of the biliary products into 

 the stomach, but closes against the entry of food into the 

 coecum. In most Gasteropoda the liver consists of two distinct 

 lobes, between which are embedded the stomach and part of the 

 intestine. In many Nudibranchiata the liver becomes ' diffused ' 

 or broken up into a number of small diverticula or glands con- 

 necting with the stomach and intestine. The so-called cerata 

 or dorsal lobes in the Aeolididae are in effect an external liver, 

 the removal of which to the outside of the body gives the 

 creature additional stomach-room. 



The Hyaline Stylet. — In the great majority of bivalves 

 the intestine is provided with a blind sac, or coecum of varying 

 length. Within this is usually lodged a long cylindrical body 

 known as the hyaline or crystalline stylet. In a well-developed 

 Mytilus edulis it is over an inch in length, and in Mya arenaria 

 between two and three inches. The bladder-like skin of the 

 stylet, as well as its gelatin oid substance, are perfectly trans- 

 parent. In the Unionidae there is no blind sac, and the stylet, 

 when present, is in the intestine itself. It is said to be present 

 or absent indifferently in certain species. 



The actual function performed by the hyaline stylet is at 

 present a matter of conjecture. Haseloff's experiments on 

 Mytilus edulis tend to confirm the suggestion of Mobius, that 

 the structure represents a reserve of food material, not specially 

 secreted, but a chemical modification of surplus food. He found 

 that under natural conditions it was constantly present, but that 

 specimens which were starved lost it in a few days, the more 

 complete the starvation the more thorough being the loss ; it 

 reappeared when they were fed again. Schulze, on the other 

 hand, believes that it serves, in combination with mucus secreted 

 by the stomach, to protect the intestine against laceration by 

 sharp particles introduced with the food. VY. Clark found that 

 in Pliolas the stylet is connected Avith a light yellow corneous 

 plate, and imagined therefore that it acts as a sort of spring to 

 work the plate in order to comminute the food, the two together 

 performing somewhat the function of a gizzard. ^ 



1 Biol. Centralbl. vii. p. 683 ; SB. Ges. Nat. Fr. 1890, p. 42 ; Mag. Nat. Hist. 

 (2) V. 1850, p. 14. 



I 



