BIOGRAPHIES OF SOME WORMS. 357 



Beneden, the development is direct, the embryo passing directly 

 into a form like the adult. 



For the further history of the fluke we will turn to Steenstrup's 

 famous work "On the Alternation of Generations," wherein is first 

 related the strange history of these animals. While the flakes 

 were well known, as well as the tadpole-like Cerearia, it was not 

 known before the publication of Steenstrup's work in 1842, that 

 the Cercariae were the free larval forms of the Distorme. The 

 Cerearia echinata, first described by Siebold, is like a Distoma ex- 

 eept that the body is prolonged into a long extensible tail. This 



• ■ , IE...-" - V 



furnished with transverse muscular fibres or stride, and between 

 each pair of these transverse fibres is placed a globular vesicle 

 wliii-h appears to be a mucous follicle or gland; the innermost 

 tube is opaque and of firmer consistence, it contains the longitu- 

 dinal muscular fibres, and is usually reticulated on the surface. 

 Through the centre of these tubes there passes a slightly narrower 

 canal, which becomes very small towards the extremity of the 

 tail. The existence of the same layers in the body itself of t • 



layer is here not so much developed. This description of the 

 Cerearia will remind one of the tadpole-like larva of the Aseidiaus. 

 The apparent homology in structure of the tail of the Cerearia 

 with that of the Ascidian larva as figured by Kupffer, is striking. 

 This similarity may be seen if the reader will compare fig. 7, 

 Tab. ii, in De la Valette St. George's " Symbola?," representing 

 a stage in the development of Cerearia flava into Monostomum 

 tiatmm. The author figures a row of cells on each side of a 

 central cavity through which passes what is regarded as pos- 

 sibly a nerve. Whether this is not as much a chorda dorsalis 

 and spinal nerve as those parts regarded as such in the Ascidian 

 larva, is a subject for future investigation. But in other respects 

 the position of the mouth, the sense-organs, as well as the form 

 of the body strikingly recall the Ascidian larva, so much so that 

 it gives strong confirmation to the opinion that the Aseidians are 

 worms, and that they and the Trematodes have possibly originated 

 from allied forms. In another species, Cerearia ocellata, the tail 

 has a lateral fin ; and in still another species figured by J. Miiller 



