138 JOURNAI^ OK ENTOMOLOGY AND ZOOLOGY 



tln-OTi,s>-h the eonnoctive tissue. The lining of the body cavity is 

 tliinnest over the hack (10 mierons), thickest over the side (30-40 

 microns) and slightly thinner over the foot than over the sides, 

 lint here it is strengthened by additional fibres in the connective 

 tissue. These form a layer just above the glandular ]iart of the 

 foot, as well as an interlacing network of fibres. Fibres also 

 branch off from the lining layer at the sides, above and below, 

 and run diagonally towards the mantle edge, towards the outer 

 angle of the foot, and into the ]>ai)illa^. 



The paiiilla^ are club-shaped processes, coming off from the 

 mantle edge in groups of various sizes. They are covered with 

 an ejiithelium similar to that covering the rest of the body and 

 contain a large branch of the mantle nerve, a large blood sinus, 

 a core of spicules, and a gland. The sinus extends along the 

 dorsal side of the papilla. At first it has a distinct wall, but the 

 upper portion breaks up into branches which communicate with 

 interstices in the very spongy connective tissue of which the body 

 of the papilla is composed. The nerve lies just venti-al to the 

 sinus. It gives off many branches in its course and finally ter- 

 minates in the gland at the tip of the papilla. The core of 

 spicules extends from the base of the impilla for about two- 

 thirds of the distance to the tip, on the ventral side. It is sur- 

 rounded by a layer of circular muscle fibres and receives the 

 fibres spoken of above as coming off diagonally from the mus- 

 cular layer of the l)ody wall. The gland of the papilla is situated 

 at the tip. It consists of a spherical mass of cells enclosed in a 

 heavy wall, and opening by a short duct surrounded by a thick 

 mass of circular muscle fibres. Nearly all of the cells of these 

 glands were empty in the preparations, probably because the 

 animal discharged the secretion when it was killed, but a few 

 contained large, dark-staining granules in a lighter, alveolar 

 mass. These full cells were pear-shaped, the nucleus being situ- 

 ated at the base at the smaller end, antl the cells were apparently 

 attached to the basement membrane by long, slender processes. 

 As the whole structure of the gland would indicate that it is 

 intended to forcibly eject the secretion, it seems likely that it is 

 used as an instrument of defense. 



