840 suEGEOiSr p. ii. welch on the anatomy or 



families the suckei's are the constant feature, and the hooklets the 

 variiible, but reversed in the Tetrarhynchs ; possibly the addition of 

 the bothria or suckers in the one is connected with the extra weight 

 of material to be anchored in the shape of breadth and thickness 

 of colony as compared w'th the other. It will be observed that I 

 have made no mention of nerve-centres in the anatom^^ of these 

 animal forms, and for the reason that, though doubtless present, I 

 am unable to distinguish any such among the components of the 

 zooids. Considering the very intricate arrangement of fibres and 

 granular material making up the parenchyma of the cephalic mass, 

 the identification and tracing the method of arrangement and dis- 

 persion of nerve-fibres and ganglia must be a matter of extreme 

 difficulty ; and, in spite of a very careful and prolonged scrutiny, 

 I cannot lay claim to any elucidation of this portion of the sub- 

 ject. The same may be said of those centres undoubtedly present 

 (by inference) in each segment of the colony. 



EXPLANATION OF THE PLATES. 

 Plates XXIV.-XXVI. 



Tetrarhynchus carcharias. Figs. 1-14. 



Fig. 1. Natural size : a, bird's-eye view of the free end of the head ; b, section 

 through the head, from before backwards, to show the position of suckers 

 and bulbs. 



Fig. 2. Natural size. Flat surface of the parasite : a, head ; b, neck ; c, colony ; 

 d, nodule at its free end ; e, anterior sucker ; /, anterior pair of pro- 

 boscides. 



Fig. 3. Magnified 15 diameters. Transverse section through a zooid at the 

 centre of the colony-length : a, skin ; h, circular and transverse mus- 

 cles ; c, inorganic layer ; d, meshwork arrangement of transverse mus- 

 cles ; e, fibrous visceral boundary ; /, visceral space ; g, cut lumen of 

 each longitudinal water-vascular canal. 



Fig. 4. Magn. 15 diam. Vertical section, from before backwards, of several 

 zooids in continuity about the centre of the colony : a, skin, showing 

 the indentations at the junction of the several zooids ; b, muscular layers, 

 circular and transverse ; c, inorganic layer ; d, parenchyma with the 

 longitudinal muscular bands ; e, visceral boundary ; /, visceral space ; 

 g, transverse water-vascular canals closely abutting upon each other. 



Fig. 5. Magn. 200 diam. Transverse section through the body-parietes : 

 a, chitinous layer of skin ; b, granular layer ; c, circular muscular fibres ; 

 d, transverse muscular fibres ; e, inorganic layer. 



Fig. 6. Magn. 200 diam. Continuation of former section to the visceral boun- 

 dary : a, meshwork formed by the transverse muscular fibres beyond 

 the inorganic layer enclosing b, the cut ends of the longitudinal mus- 

 cular bands and the granular albumenoid material of parenchyma ; c, 

 fibrous boimdary of the visceral space. 



