1902.] ORIGIN OF PEARLS. 151 



If it presses upon the tissues intervening between itself and the 

 shell, these may become absorbed, in which case the epithelium 

 of the pearl-sac becomes continuous with the shell-forming epi- 

 dermis (text-fig. 22). The result is that the subsequently formed 

 layers of the pearl are continuous with those of the shell, and an 

 attached pearl is formed. The fi;sion of two or more pearls to 

 form a compound pearl is effected in the same way. 



Structure of the Trematode Larva. 



With the exception of the female reproductible organs, which 

 are as yet undeveloped, the larva presents all the characters of 

 Bistoinutn [Brachycoelmm Dujardin, 1845, Leucithodendrium Loos, 

 1896) somaterice (Levinsen, 1882), from the Eider Duck, Greenland. 

 (For sub-classification of Dujardin's subgenus Brachyccelium, see 

 Stossich, 1899.) The body (PI. XV. fig"! 6) is oval, blunter in 

 front than behind, and tapering markedly in the last third. In the 

 resting condition inside the sac it is nearly spherica.1. The average 

 dimensions are "55 mm. to "7 mm. The extreme sizes seen were 

 •45 mm. and "75 mm. The oral sucker is larger than the ventral 

 one, the ratio of their diameters being usually about 4 : 3. Bvxt 

 in this point there is a considerable amount of variation both in 

 the larva and in the adult L. somaterice. Except on the surfaces 

 of the suckers, which are smooth, the cuticle is beset with small 

 spines (text-fig. 23). These are arranged in transverse rows, the 



Text-fig. 23. 



W ¥ Y V Y V Y r¥ VI 



¥ \^ Y Y T'1 V f ¥ f 



y Y Y f ¥ ¥ V V HP 



Cuticle of the Cercaria, in surface view. X 700. 



members of which also form diagonal rows, so that the cuticle 

 in surface view appears to be divided up into little diamond- 

 shaped fields. There are about two hundred transverse rows of 

 spines on the dorsum. Immediately around the ventral sucker 

 the spines occur in concentric circles. The connective tissue is 

 the typical parenchyma of the Flatworms (PL XIV. fig. 2 ; PI. XV. 

 figs. 5, 7, pa.). It is richer in nuclei immediately under the skin 

 than elsewhere, especially on the dorsal surface (PI. XV. fig. 7, 

 pa.n.). Some muscle-fibres are present in the connective tissue 

 running from the body-wall to the suckers and pharynx (PI. XV. 

 fig. 7, m.f.). The musculature of the body- wall consists of an 

 outer circular layer (PI. XV. figs. 5, 7, cm.) immediately under 



