ANIMAL PARASITES. 63 



four vessels rising in a tortuous form through the upper part of 

 the lobe, and passing over on the external sac-like envelopes, are 

 also observed ; but nothing is known of their ramifications. 



The vesicle now constantly becomes larger and clearer, its 

 contents becoming more fluid, and the medullary layer displaced 

 as far as its peri ph eric layers, the epidermis and muscular layer 

 grow thicker, whilst vessels and calcareous corpuscles 1 make 

 their appearance, especially in the circumference of the anterior 

 part of the body, in the stratum between the muscular and 

 epidermic layer, but not in the muscular layer, as stated by 

 Wagener, in some cases before, in others after, the formation of 

 the cephalic process. The vessels of this stratum, formerly re- 

 garded as a net of muscles by Pallas, and overlooked until the 

 time of G. R. Wagener, are very strong, and surround the caudal 

 vesicle with dendritically ramified stems (thin- walled tubes), 

 uniting directly with the vessels of the head, which probably 

 originate from them. In the vessels, which probably serve for 

 the purpose of excretion, circulates a colourless, limpid fluid, 

 which is set in motion by cilia. The latter are either whip-like 

 hairs, or semicircular lobes, with a free margin, moving from one 

 side to the other, which cannot be decided easily ; according to 

 Leuckart, they are situated only in the smaller vessels, although 

 I believe I have seen them also in the larger branches, in points 

 of curvature, and especially where one vessel opens into another. 

 These ciliary structures were discovered by Lebert, and afterwards 

 particularly described by Virchow and Wagener. That these 

 vessels unite into a pulsating tube, opening at the end of the 

 caudal vesicle (Wagener), could not be detected either by Leuckart 

 or the author. Neither could Leuckart find any openings of the 

 large vessels of the head towards the interior space of the caudal 

 vesicle, but he sometimes found two openings on the neck, by 

 which the longitudinal vessels are supposed to lead outwards. 



1 Even the calcareous corpuscles have their own proper fragment of history. Once 

 they were regarded as eggs ; subsequently their calcareous nature was discovered, and it 

 was agreed that they consisted principally of carbonate with a very small proportion, 

 perhaps, of phosphate of lime. Some, such as Eschricht, regarded the corpuscles which 

 dissolve with difficulty in acids as siliceous globules, but all believed them to be a 

 normal phenomenon, and the first indication of the formation of a skeleton in animals. 

 Huxley now suddenly declares them to be of a morbid nature ('Annals of Nat. Hist.,' 

 xiv, 1854). There is equal reason, as Leuckart says, to call the calcification of the 

 skeleton morbid. 



