LINNEAN SOCIETY OF LONDON. 23 



unsatisfactory condition, viz. : — (1) the supposed transference of 

 the parasite from another Molhiscan host {Tapes or the Cockle) 

 to the Musse], and (2) the mode of origin of the epithehal sac 

 which encloses the larval parasite and secretes the pearl. As 1 am 

 discussing these two points in some detail in the forthcoming 

 Part V. of my Ceylon lieport, I shall not go into the matter here 

 beyond saying that Jameson, although drawing attention to the 

 similarity between the epithelium of tlie pearl-sac and that of the 

 outer surface of the mantle, evidently considered that the two 

 layers are not genetically related. He states definitely in regard 

 to the peai'l-sac, " this epithelium appears to arise quite indepen- 

 dently of the outer epidei'mis." Boutan, in Prance, has written * 

 controverting this independence, and contrasting Jameson's 

 view of a mesodermal origin with his own view (which, I may 

 add, I entirely agree with) that the two epithelia are genetically 

 related, and that the pearl-sac must arise from the exactly similar 

 cells of the ectoderm. It is highly probable that the parasite in 

 burrowing into the mantle carries in with it one or more epidermal 

 cells which proliferate to form the sac. As the Distomid larvse 

 are found moving on the inner surface of the shell before coming 

 to rest in the mantle, they must traverse the epidermis, and it is 

 natural to suppose that in their migration they may push some 

 epidermal cells in before them. Even in the absence of direct 

 evidence of this (and we have some evidence) it will be admitted that 

 the process does not involve such a violent assumption as that 

 the connective tissue in the centre of the mantle can pi'oduce an 

 epithelial sac, the cells of which are indistinguishable both in 

 structure and in function from the epidermis outside. 



In order that I might make certain how far our views really 

 differed, I thought it A\orth while lately to exchange letters on the 

 matter with Dr. Jameson, who is now in charge of the Biological 

 department of the Transvaal Technical Institute at Johannesburg; 

 and his answer, received a week or two ago, contains the following 

 passage, which he evidently wishes me to make public : — " I am 

 very glad to have the opportunity of clearing up, through you, 

 the uncertainty caused by my unintentionally ambiguous state- 

 ments re origin of pearl-sac epithelium. I had never any doubt 

 that it is a true epidermis, but I never got so far as to determine 

 actually by observation whether it arose, as I think you have 

 suggested, by the Trematode carrying in with it a fragment or 

 pocket of epidermis ; or, as I suspected, by means of epidermal 

 or sub-epidermal replacement cells (Ersatz-Zellen) which are known 

 to occur in many invertebrates, often in sub-epidermal tissues, 

 and which replace epidermal cells if they are injured. I left the 

 question open in my paper because I hoped to make some experi- 

 ments on the nature of the epidermal cells ; indeed, I had actually 

 started these experiments when the temporary breakdoAvn of ray 

 health necessitated my dropping the work, and as you know, 

 * Arch. Zool. Exper. s6r, 4, t. ii. (1904). 



