Contributions to the Anatomy and Histology of Thalassema neptuni Gaertner. 537 



on the proboscis, as also in the intervals between the papillae on the 

 body. It stains deeply with Bismarck brown, but is best seen in pre- 

 parations treated with crude pyroligneous acid ( 24 h" ), and osmic 

 acid ( 24 h""^ ). In the cuticle the following strata can be distinguished : 

 1) a deeply staining and highly refractive outer layer which in 

 sections appears as a dark border (Fig. 2 a cu') ; 2) a clearer, slightly 

 granular layer, which at times shows a faintly reticular structure in 

 section (Fig. 2a cu); and 3) an alveolar layer which measures about 

 V4 of the total thickness of the cuticle , and connects it with the 

 epidermal cells (Figs. 1, 2, 2 a and 6 al). This layer, which may be 

 regarded as a transition from the protoplasm of the epidermal cells 

 to the cuticle consists of a single stratum of vacuoles bounded ex- 

 ternally by the cuticle and internally by the protoplasm of the epider- 

 mal cells. Over the "clear cells", to be mentioned below, the 

 alveoli sometimes appear somewhat larger, and the cuticle often shows 

 a tendency to become thinner. This stratum appears on surface 

 section highly vacuolated. The middle layer of the cuticle is also by 

 no means structureless as sometimes stated; a finely reticular 

 structure being plainly visible in surface sections and macerated pre- 

 parations, when examined under a high magnifying power (Fig. 3). 

 The meshwork of this reticulum shows a marked tendency to form a 

 system of regularly intercrossing lines (Fig. 3). This is comparable 

 to the well known structure of the cuticle in earthworms; and to 

 that described by Andreae in Sipunculus (1, p. 207), where the meshes 

 are likewise not absolutely regular. Such a structure has also been 

 described in other Sipunculoids {Phascolosoma, Bütschli, PhascoUon; 

 Théel). On the proboscis ^ and more rarely on the body wall the 

 outer surface of the cuticle is seen to be divided up into little 

 quadrangular fields; this curious marking is superficial and has no 

 apparent relation to underlying structures. The ducts of the great 

 unicellular glands open between the polygonal outlines of the 

 epidermal cells (Fig. 4) ; they are very narrow as a rule, and generally 

 show a slightly funnel-shaped outer opening. The cuticle resembles 

 that of other worms in its ready solubility in boiling caustic potash. 

 The epidermis (Figs. 1, 2, 5, 6) consists of an epithelium which 

 is much modified by the functional differentiation of its cells. It is 

 ciliated only on the ventral surface of the proboscis. We may recognise 

 at least four kinds of cells : 1) typical cylindrical epithelial cells (ep. c) 

 the outer ends of which form the pavement on surface view (Fig. 4), and 

 which alone seem to enter into a close connexion with the cuticle ; 2) clear 



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