ON THE DEVELOPMENT OF THE POND-SNAIL. 369 



organ, but shall refer to the woodcuts (figs. 1 and 2) as illus- 

 trating its position and character in the embryos of Pisidium 

 and of Aplysia {Pleurohranchidium) respectively. The 

 gland {shs) under certain circumstances, connected with 

 an arrest of regular development, becomes filled with a 

 chitinous plug in the case of the Aplysia embryo. When 

 at Messina, during May of this year, I found that Dr. 

 Herman Fol had discovered the same shell-gland in embryo 

 Pteropods, and, strangely enough, he had found the same 

 plugging with a chitinous secretion in specimens abnor- 

 mally developed. I have observed the same " shell-gland" 

 in an early stage of Neritina, and, as will be seen below, it 

 has a very well-marked development, accompanied by occa- 

 sional plugging with a chitinous material, in LymncBiis. 



The position of the gland in Pisidium, and its relation to 

 the pair of ca,lcareous valves which develop on either side of 

 it, suggests that it may in the Lamellibranchs be repre- 

 sented in adult life by the ligament; but this connection I 

 have not been able to demonstrate ; on the other hand, in 

 Aplysia, Neritina, LymncBus, and the Pteropods, it certainly 

 disappears — is, in fact, an evanescent embryonic structure. 



One naturally turns, after detecting this organ in Lamelli- 

 branchs, Gasteropods, and Pteropods, to the classes which 

 have been (I think a little invidiously) separated as Mollus- 

 coida from the other Molluscs — I mean the Brachiopoda and 

 the Polyzoa — to see if in them any trace of the shell-gland 

 can be found. I do not know, at the present moment, of any 

 such organ having been as yet observed in the young stages 

 of Polyzoa. But in a very strange form, which must be 

 classed with the Polyzoa, there is such an organ, occupying 

 exactly the required position. 



Lozosoma neapolitanum was described first by Keferstein, 

 and subsequently by Kowalewsky, from whose memoir the 

 accompanying woodcut (fig. 4) is taken. The large gland 

 of attachment {shs) appears to me to be very probably the 

 homogen of the shell-gland. Further, in the Brachiopoda we 

 have a gland developed at the same point in many forms, ap- 

 pearing at a very early stage in Terehratula and Trebratulina, 

 and well known as enabling the animal to fix itself by means 

 of its pedicle. The position of this gland corresponds accurately 

 with that of the shell-gland in the embryo Pisidium, Aplysia, 

 and Lymnaus. Hence I consider that we have evidence for 

 considering this organ as one common to Polyzoa, Brachio- 

 poda, Lamellibranchia, Gasteropoda, and Pteropoda. 



A question which at once presented itself after the general 

 presence of this organ had been ascertained was this — Does 



