ENTOZOA. 75 



it is seen to be a nucleated cell. In this are developed two cells, 

 which are liberated, and, repeating the process, form four* cells ; 

 this cluster of germ-cells divides the more opaque yolk mechanically 

 into two parts. The germ-cells become progressively smaller and more 

 numerous, and the yolk becomes in the same ratio acted upon and 

 diminished, until it is wholly absorbed and assimilated by the germ- 

 cells, and the egg finally contains only the germ-mass so constituted ; 

 that is, consisting of the progeny of the primary germ-cell, de- 

 veloped and multiplied at the expense of the yolk, which is, 

 therefore, exclusively a " germ-yolk " in the Tcenice. The embryo 

 above described is the result of the metamorphoses of certain cells of 

 the germ-mass into its different tissues ; viz. the skin, the proboscis, 

 the six booklets, and the suckers ; the contained parenchyme consists 

 of the remnant of the germ-mass comparatively little changed. 



The next step in development has been observed by Siebold in 

 the larvae of TcBnicB, which have become parasitic in 

 animals of quite distinct species from those in which 

 the ova and ovigerous segments were formed. 



In a species of slug {Ario7i empiricorum), e. g., Sie- 

 bold observed minute white cysts projecting from the 

 inner surface of the pulmonary sac. Each cyst con- 

 tained a vermicule, with an uncinated and suctorial 

 head retracted within the body. By regulated pres- 

 sure the head was everted, and also the opposite end 

 of the body, which had previously been intussuscepted, 

 and the animalcule then presented the form given in 

 Jig. 33. Instead of six hooks, these were now ar- 



Larva of Tania ^^^S®^ ^^ ^ doublc roW of ten in each. The con- 

 magnified. ' tractile parenchyme of the unjointed body shows 

 the vascular system above described, together with numerous minute 

 cells and the clear calcareous corpuscles, but no trace of genera- 

 tive organs. These sexless larval cestoids are probably excluded 

 from the ova of the taeniae of some species of bird, voided with 

 the excrement. Being hatched, they creep upon the body of the 

 first slug that may crawl near them, and entering the open orifice of 

 the pulmonary sac excite a certain inflammation by their hooks, 

 become surrounded by an adventitious cyst, and attain that grade of 

 development, as manifested by the number of the hooks, the suckers, 

 and the length of the body, by which they differ from the larvae as 

 first formed in the egg of the tape-worm.* They have never been 



* With regard to the change from the six-uncinated embryo to the larvas with 

 the double crown of hooks, Stein states that the latter are a new formation which 

 supersedes the embryonal armature of the proboscis. LXVII. p. 69. 



