738 SUMMABY OF CURRENT RESEARCHES RELATING TO 



spends to the arm of the lever, its power would be reduced, were it not for 

 compensation involved in an immense increase of volume and a most efficient 

 insertion on specifically peculiar muscle apophyses. Similar adaptations 

 are well known in other animals. 



There are also secondary modifications associated by correlation of 

 organs with the primary. The strengthening and approximation of the 

 two shell-muscles involves a considerable reduction of the space between 

 them. It is therefore natural that the organs in this region are modified 

 by mutual pressure. By a shortening of its longitudinal axis the ventricle 

 of the heart comes to lie quite transversely, the dorsal vessel exhibits a 

 sharp bend, the posterior aorta being forced to pass for some distance in 

 front of the ventricle, so as to reach along with the rectum (which penetrates 

 the ventricle inferiorly and posteriorly) the upper surface of the posterior 

 adductor. The auricles remain approximately equal in length, but their 

 form is modified by the lateral horns of the kidney, which are insinuated 

 between the former and the base of the gills. The same position of the 

 organs occasions the restriction of the openings of the vasa revehentia 

 branchiarum to a very restricted portion of the anterior end of the wall of 

 auricle. That portion of the kidney which represents the typical pair of 

 tubes in the most nearly related forms is also compressed longitudinally, 

 so that it forms a simple transverse sack, the "central portion." This 

 restriction of the secreting area must be compensated for in some way. This 

 can only occur by the development of diverticula between the adjacent 

 organs. There are two pairs of such structures extending forward from the 

 central portion. The dorsal pair are insinuated between the posterior 

 adductor and the pericardium forward to the rectum ; the two lateral horns 

 of the kidney lie between auricle and base of gills. In these the glandular 

 epithelium is abundantly folded inwards. The shortening of the ureter is 

 not, however, compensated for in any way, length being for such an organ 

 of no moment. In the nervous system, the centres of which lie before and 

 behind the region of most modification, the effect of shortening is only 

 seen in the relatively slight length of the commissure between cerebral and 

 visceral ganglia. In the gills, on the other hand, the marked shortening 

 of the lamellae is probably made up for by an increase in thickness. The 

 shortening is also associated with the appearance of a special membrane 

 which completely separates the mantle cavity into anal and branchial 

 chambers. The third adductor of the shells, which has been differentiated 

 from the pallial muscle, is also conspicuously short. The other organs, 

 such as stomach, intestine, enteric appendages, central portions of nervous 

 system (with the exception of the enigmatical small ganglion in front of the 

 visceral) are not within the range of the most modified region, and show but 

 slight deviations from the ordinary Pholas type. 



The form of the accessory shell-pieces ought also to be interpretable 

 in relation to external conditions. The shape of the shortened shell of the 

 young form is of a skullcap-shape ; the young animal can bore a perfectly 

 round hole in the coral block ; this dwelling-place determines the form of 

 the fresh pieces. The shell of the adult must complete the sphere. 



The mobile larva, the young boring form with toothed shell and 

 muscular foot, the sexually mature adults, all exhibit modifications cor- 

 responding to their different conditions. These are discussed in the course 

 of the memoir. 



Jouannetia cumingii is connected with the more typical forms by a series 

 of transitional stages. It stands itself as the extreme of a long row of 

 forms. Pholas dactylus and similar forms may be taken as primitive types 

 of the open Pholadidse. The closed types pass in their young forms 



