ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 561 



ment of its muscular fibres. In this connection an investigation must 

 be made into tbe morphology of the Pteropoda ; in them, too, there is a 

 great differentiation of the foot, leading to the formation of two laterally- 

 placed fins — the epipodium of Huxley — and, in some, to the distinction 

 of a horseshoe-shaped and a conical piece in the median part of the foot. 

 As in Heteropods, the fin-musculature of Pteropods is formed by rays 

 from the spindle-muscle ; by this character and by the independence of 

 the whole fin-musculature of the musculature of the body, the fins of 

 Pteropods are shown to be homologous with those of Heteropods. 



It has not yet been demonstrated that these fins are derived, onto- 

 genetically, from the foot. The protopodium of all Molluscs is a mere 

 outpushing of the body-wall into which the coelom is merely continued. 

 In the Heteropoda this protopodium is separated from the body by the 

 caudal portion and carried backwards. This caudal portion is a structure 

 quite similar to the protopodium, in so far as it is a mere prolongation of 

 the body-wall, although filled by gelatinous material. The intercalation 

 of the caudal portion may be regarded as due to the growth of the tissue 

 at the base of the protopodium, and this region may be looked upon as 

 part of the organ which corresponds to the developed protopodium. If 

 this be so, there is nothing surprising in the musculature of the body 

 passing directly into the tail. The arrangement of the muscles of the 

 fin seem to show that it is not a differentiation of the protopodium, but 

 a formation sui generis. While we regard the tail as an outgrowth of 

 the body, due to local growth, the fins of Heteropods and Pteropods 

 must be looked upon as an outgrowth of the spindle-muscle, or of a part 

 thereof; the body- wall having been broken through in such a way that 

 the newly-formed structures are only accompanied by the epidermis and 

 the gelatinous cuticle. 



When we ask if there is in Gasteropods or Lamellibranchs any organ 

 homologous to the fins of Pteropods or Heteropods, we find that in them, 

 as in all Molluscs save Cephalopods, the first rudiment of the foot is the 

 protopodium, which is the only differentiation on the ventral surface of 

 the embryo. No other differentiations appear, or, in other words, there 

 is no deutopodium. 



y. Gastropoda. 



Abnormal Growth in Haliotis-t — Mr. E. A. Smith gives a descrip- 

 tion of an example of the Japanese Haliotis gigantea, which is remarkable 

 for having two rows of perforations in the shell instead of one. Four of 

 the holes of the outer or normal series are open, while all those of the 

 inner series are closed or filled up. Mr. Smith supposes that the edge 

 of the mantle at this particular point was accidentally notched in early 

 life (or from congenital defect), and that the notch was not deep. It is 

 probably correct to suppose that the perforations are for the purpose of 

 conveying water to the gills, and to some extent, for the extrusion of 

 fasces. As there are neither gills nor anus beneath the abnormal series 

 of holes, they had no special function to perform, and so became closed 

 up as soon as possible. In figures of H. tuherculata given by Cuvier and 

 by Fischer a tentacle may be seen to be protruded through each of the 

 last six or seven perforations ; in no specimen or species examined by 

 Mr. Smith are there ever more than three tentacles, and these are always 

 similarly located. 



* Ann. and Mag. Nat. Hist., i. (1888) pp. 419-21. 



