CESTODA 117 



membranes ; an outer, thick, laminated, homogeneous elastic 

 layer (the ectocyst of Huxley), and an internal, thin, soft, granu- 

 lated, comparatively inelastic layer the endocyst of the same 

 author. The terms are convenient. The ectocyst is structure- 

 less, consisting of a substance closely allied to chitine. For this 

 and other reasons it has been called the cuticular layer, but 

 the endocyst is the essential vital part of the animal, repre- 

 senting a huge compound caudal vesicle. In an hydatid from 

 the zebra, Huxley found that the endocyst was " not more than 

 ~th of an inch in thickness, being composed of very delicate 

 cells of j !joo" to 3^-" in diameter, without obvious nuclei ; but 

 often containing clear, strongly refracting corpuscles, generally 

 a single one only in a cell/' Prof. Huxley adds : " These 

 corpuscles appear to be solid, but by the action of dilute acetic 

 acid the interior generally clears up very rapidly, and a hollow 

 vesicle is left of the same size as the original corpuscle. No 

 gas is developed during this process, and sometimes the 

 corpuscles are not acted upon at all by the acid, appearing then 

 to be of a fatty nature. A strong solution of caustic ammonia 

 produces a concentrically laminated or fissured appearance in 

 them. Under pressure and with commencing putrefaction a 

 number of them sometimes flow together into an irregular or 

 rounded mass." 



The precise mode of development of the echinococcus-heads 

 or scolices has been a subject of lengthened discussion between 

 Leuckart and Naunyn. According to Leuckart the earliest 

 indication of the scolex consists of a slight papillary eminence on 

 the inner surface of the granular endocyst. After a short 

 period this prominence displays in its interior a vacuole-like 

 cavity, the latter being occupied, however, with a clear limpid 

 fluid. Its margins become more and more clearly defined, until 

 the cavity is by and by seen to be lined with a distinct cuticular 

 membrane. The papilla increasing in size, becomes at first 

 elongated or oval, eventually scoleciform, or even, perhaps, a 

 true echinococcus-head. Thus far the description bears out, in 

 a measure, the theoretical notions entertained by the older 

 authors ; but the developmental process does not stop here. The 

 scolex-development has now to sacrifice itself by developing in 

 its interior a brood of scolices or echinococcus-heads. In other 

 words, it becomes transformed into the so-called brood-capsules 

 of Leuckart and other authors. These structures were pre- 

 viously well known to Professors Erasmus Wilson and George 



