f 



8 ZOOTOMY. 



behind the corresponding gill-cleft. The longitudinal connecting bars 

 (k and i) are flattened, and unite with the third (c) and the fifth (e) 

 segments of the transverse arches respectively. In front of the first gill- 

 cleft the two longitudinal connecting bars run together and form a 

 single rod, which becomes connected with the first transverse arch (k). 

 This latter is free at its dorsal end, of tolerably regular cylindrical form, 

 and united by a short rod of cartilage with the proximal end of the 

 styliform process (st.p). The eighth transverse arch is also regularly 

 cylindrical at its dorsal and ventral ends, but in the middle of its 

 course becomes fused with the hinder ends of the longitudinal connect- 

 ing bars and with the pericardial cartilage (pc). The latter has the 

 form of a backwardly directed hemispherical cup, supporting the 

 posterior wall of the pericardium, and produced into dorsal, ventral, 

 and lateral processes, of which the last enter into union with the bran- 

 chial basket proper, as already described. 



The cartilages of the branchial basket lie, for the most part, immedi- 

 ately beneath the dorsal and ventral muscles covering the gills ( 34, Fig. 

 6, h, z), so that they are seen at once when those muscles are removed : 

 but the pieces marked b, d, and f, are curved inwards and are con- 

 sequently not seen until the surrounding tissues are dissected away. 1 







B. Directions for Dissection. 



III. Verify the following external characters :- 



19. The elongated vermiform body, almost circular in 

 section anteriorly, but becoming compressed from side to 

 side posteriorly ; the head passes insensibly into the trunk, 

 and the trunk into the tail. 



20. The integument, smooth and slimy, and entirely 

 devoid of scales : marbled with black in P. marinus, of a 

 uniform bronze-green tint in P. fluviatilis. 



21. The sucker-like oral funnel (Fig. 4, o.f) at the 

 anterior end of the body, inclined obliquely downwards and 

 forwards : its rim, which is beset with numerous vascular 

 papillae, passes insensibly into the general surface of the 



1 This description of the branchial basket is taken from a single 

 specimen of P. marinus the only one at my disposal. 



