24 W. BALDWIN SPENCER. 



in the figure, each one is seen to be columnar with a somewhat 

 larger club-shaped head, internally containing a distinct round 

 or oval nucleus, whilst the more pointed external end passes to 

 the stigma. The latter has a definite wall^ which is doubtless 

 a special modification of the cuticle, but appears distinct from 

 this in section. Just at the mouth of the gland the wall of 

 the stigma is thrown into a slight fold (-R.), and from this a 

 thin layer appears to pass inwards, which forms the external 

 boundary of the gland. This cannot often be detected as a 

 distinct continuous structure, but traces of it can be seen, and 

 the very definite form of the gland indicates the presence of 

 some such bounding structure. 



The stigma is hour-glass shaped, and its external opening 

 projects as a raised lip beyond the surface of the cuticle. 



Whilst there can be no doubt that the cells constituting the 

 glands are modified subcuticular cells, it must be noticed that 

 each one is much larger than one of the latter, and that the 

 whole gland projects considerably beyond the level of the inner 

 end of this layer. At times the cells of the glands contain a 

 large amount of protoplasm, at times only a protoplasmic 

 network is present, doubtless owing to the fact that fluid 

 material has been secreted and passed out. 



The second group comprises the remaining secretory glands 

 of the animal, all of which are intimately associated with one 

 another. The cells composing them are identical in structure, 

 and are in all probability to be regarded as mesoblastic in 

 origin. Ducts, when such are present, are, on the other hand, 

 lined with a definite cuticle, and to be regarded hence as epi- 

 blastic in origin. 



For the sake of convenience we may deal with these struc- 

 tures under the following names : 



(1) Hook-gland (=hook-gland of Leuckart in P. pro- 

 boscideum and P. oxycephalum, hook-gland of Hoyle in 

 P. protelis, and head-gland of Stiles in P. proboscideum). 



Either of the names hook- and head-gland is applicable to 

 this structure, so far as P. teretiusculum is concerned, if 

 we simply pay attention to the external opening of the gland ; 



