134 ENTOZOA. 



The accompanying drawings (Plate IX.) represent a scolex or 

 larval tetrarliyncli, wliicli I removed from the wall of the intestine 

 of a common haddock. It was associated with several other larvae 

 of the same kind, the individuals varying in size from a pin's head 

 to that of a pea. Each young Tetrarhynchus was enclosed in a 

 thick-walled sac (fig. 1), wliich, when burst open, set the larva 

 free. The sac itself is made up of two distinct layers, an inner 

 smooth one (endocyst), and an outer fibrous layer (exocyst) con- 

 sisting of dense bands of connective tissue. Immediately on its 

 escape the larva is seen to consist of an oval mass, at the surface 

 of which a few transverse folds are plainly discernible, and also a 

 number of minute dots which are visible with a low magnifying 

 power (fig. 3). "Within the vesicle a second round and much 

 smaller vesicle is distinguishable, and within this latter, again, 

 distinct traces of the head are rendered apparent by the aid of 

 transmitted light. On pressing the larva between two glasses the 

 inner vesicle readily protrudes at one end of the mass, and, by 

 carrying the pressure still further, the head and neck themselves 

 may be everted (fig. 4). The large outer bladder-like vesicle, is, in 

 fact, the tail of the scolex ; the second vesicle being only its anterior 

 pole inverted, and enclosing the introverted head and neck. Whilst 

 still within the caudal vesicle, the four long filamentary proboscides 

 were readily discernible, whilst the dots, just alluded to, are the well- 

 known calcareous corpuscles. When highly magnified these several 

 parts became much more significant. The head, of uniform 

 breadth with the neck, supported four quadrangularly disposed 

 leaf-hke appendages, each of these organs being pointed at the free 

 end and slightly hollowed out at the upper surface to form the 

 characteristic bothrium (fig. 5). At the base of each appendage 

 (auricle) is a minute papilla- shaped opening, through which a pro- 

 boscis is darted out somewhat after the fashion of a projectile. 

 The four openings lead into four long, cylindrical, muscular 

 channels, in which the proboscides were lodged in their retracted 

 condition. These sheaths extended all the way down the neck to 



