Respiration in Invertebrate Animals. 255 



as a starting-point of great value in the determination of ana- 

 logies and homologies in the lower members of the scries. This 

 demonstration is reserved for another o])])ortunity. 



A second and much smaller gland exists in the respiratory 

 cavity of the Pectinibranchiata, which hitherto has been variously 

 called the " colour-gland " and the bipectinate supplementary 

 gill. This body is not present in the breathing-chamber of the 

 Pulmonata. In Helix a fringe-like fold (fig. 3 e) of the mem- 

 brane of the cavity assumes almost the a])pearance of a gland ; 

 it is however nothing but a portion of the vascular respiratory 

 membrane. 



The so-called colour-gland is best studied in Buccinum and the 

 Littorinida. In both it is situated on the extreme of the roof of 

 the cavity, being separated from the other gland by the inter- 

 position of the branchise. It is considerably smaller in the 

 Periwinkle than in the Whelk. It has a dark green colour. It 

 commences posteriorly in a csecal extremity. It is prolonged 

 anteriorly into a tube or duct which travels underneath the 

 mucous membrane of the vault until it approaches the termination 

 of the rectum, where it has its outlet by a separate orifice. 

 Viewed as an axis (fig. 9 a, a), this duct may be described as 

 supporting the lobes or leaves {b, b) of the gland, — as symme- 

 trical, bdatcral, ramose diverticula. This gland presents a 

 general exterior resemblance to the so-called " muciparous 

 gland " of this chamber. It admits of division into two parts — 

 the lobes first, which correspond with the laminse ; and, secondly, 

 the lobuli (c) into which the lobes (/;, b) are further subdivided. 

 One of the lobuli in minute structure represents the arrangement 

 of the whole gland. These lobzili do not exist in the " muci- 

 parous gland." This is one distinctive fact. The next is that 

 the latter gland has a yellow colour, the former is of a dark 

 green. But distinctions more essential than the preceding 

 remain to be indicated. A lobule is represented by a bunch of 

 grapes Jlatiened (fig. 10). The grapes or terminal f'olUclcs {a, a) 

 do not exhibit the same figure or shape as they do in the 

 " muciparous gland ;" they are more elongated and conical. 

 From the latter they differ also in their contents. They cir- 

 cumscribe cells which cannot be confounded with those of the 

 muciparous gland of the respiratory cavity. The stromatous 

 tissue (fig. 10 b) which envelopes the caca is obviously dissimilar 

 from that of the latter gland. TJie dark green colour (c) of 

 both the extracsecal and intracsecal cells is one striking fact of 

 distinction. This colour is seated in the nuclei of the cells. Like 

 those of the " muciparous gland," these cell-elements are divisible 

 into two groups ; those, first, which are external to the caecum ; 

 and, secondly, those which are within {d) . Those which are with- 



