22 W. r>ALDWIN SPENCEE. 



excretory nature. Though they have been described in various 

 ways and under slightly different names by different workers, 

 there appears to be on the whole a fundamental agreement in 

 the species examined with regard to the distribution and 

 arrangement of the gland masses, though at the same time 

 they are more extensively developed in some than in others. 



Leuckart described in P. tsenioides (1) stigmatic glands 

 and (2) hook-glands. Hoyle, in P. protelis, described (1) 

 hook-glands, (2) parietal cells, and (3) stigmatic cells; whilst 

 Stiles, in P. proboscideura, has recently described (1) stig- 

 matic glands, (2) parietal glands, (3) head-glands, (4) hook- 

 glands, together with masses of cells round the oesophagus 

 and rectum. 



There is a certain amount of confusion in the names, since 

 what Hoyle has described as "hook-glands" in P. pro- 

 telis are the undoubted homologues of the structures to 

 which the same name had been given by Leuckart in P. pro- 

 boscideum, which are present inLinguatula Diesingii 

 (van Beneden) ; whilst to the homologous structures in P. 

 proboscideum Stiles gives the name of " head-gland." 



In P. teretiusculum the arrangement of the gland 

 masses is very similar to that of P. proboscideum, and by 

 interchanging the names "head-" and "hook-glands," as 

 used by Stiles, they may be directly compared with those 

 described by Leuckart, Hoyle, and other investigators. 



These secretory structures fall into two groups, which are 

 quite distinct in form and function from each other. The one 

 set, comprising the stigmatic glands only, are epiblastic in 

 origin, and simply modifications of the cuticle-forming layer 

 of cells; whilst the others are more deeply lying structures, 

 and derived presumably from mesoblast. The one set is to be 

 regarded as secreting a material useless to the animal, at all 

 events in the adult state ; the other as secreting a fluid of con- 

 siderable importance to the parasite, the nature of which will 

 be suggested later. 



The first group comprises only the stigmatic glands. 

 These appear to be present in greater numbers in P. tereti^ 



