MORPHOLOGY OF THE HELICINID^. 773 



the oesophagus, and curves round it till it nearlj- touches the 

 pyloric end of the stomach again ; here it passes into the large 

 intestine, which turns sharply back, passes round the oesophagus 

 again, and coming to the sui'face sweeps round to the left in front 

 of the anterior corner of the pei'icardium to form the descending 

 loop of the rectum. This type is easily derived from the first by 

 the shortening of both the small and the large intestine, in 

 consequence of which the former is hooked round the oesophagus 

 and one of the bends characteristic of the first type is suppressed. 

 The third type, seen in EiUrochatella ^mlcliella (fig. 15), difi'ers 

 considerably from the other two. The small intestine is even 

 shorter than in Lucidella, and the loop formed by the large 

 intestine and the first section of the rectum lies wholly on the 

 dorsal side of the oesophagus ; this condition is clearly due to 

 the gut being much shorter than in the other types, and it 

 appears to be quite a constant feature in Eutrochatella. The 

 intestinal coils of Aphanocorda andmnanica are singularly like 

 those of Eutrochatella. 



I may appear to be giving an undue amount of attention 

 to characters of no obvious morphological or physiological 

 importance, but it is just because they may be claimed to be 

 of importance in the economy of the species that I have spent a 

 considerable amount of time in working out these details. Each 

 species seems to have a characteristic arrangement of the coils 

 of the intestine, and the arrangement is remarkably constant in 

 individuals of the same species, allowance being made for 

 displacements due to the greater or less state of contraction of 

 the specimens. Closely allied species, such as Alcadid palliata 

 and A. hollandi, have a very similar arrangement, yet sufficiently 

 diff"erent to allow one to recognize them at a glance after 

 obtaining some familiarity with their anatomy. Lucidella and 

 Eutrochatella, both separated from Alcadia and from one another 

 by distinctive characters of shell, operculum, and radula, diffei- 

 in a nearly corresponding degree in the coils of the intestine. 

 Orohophana and Palceohelicina are Pacific foi-ms which must 

 have been derived from American Helicince, the latter being 

 closely related to Alcadia., and they resemble the last named in 

 the coils of the intestine. To this extent it may be claimed that 

 two, three, or more characters vary together in these genera ; but 

 Aphanoconia presents a difficulty, for this genus is far removed 

 from Eutrochatella in shell and radular characters, and is in these 

 respects closely related to Palceohelicina, 3^et its intestine is as 

 nearly as may be that of an Eutrochatella. As the two genera 

 cannot possibly stand in close genetic relationship to one another, 

 the similarity in the pattern of the intestinal coils must be due 

 to parallelism, similar causes producing similar deviations from 

 type in the two organisms. It has been shown that the 

 differences in pattern are attributable to differences in the 

 length of the large and small intestines, and this is probably 

 connected with different forms of food. As we are ignorant of 



