Respiration in Invertebrate Animuls. 311 



round the circumference of the process ; C, corpuscles of the 

 blood. 



Fig. 7, B. A single pair (magnified and viewed by transmitted light) of the 

 branchial leaflets of the Hermit Crab : c, d, two longitudinal 

 blood-channels, seen in section ; f, g. embrace a deep groove be- 

 tween the leaflets for the branchial current of water ; t, A. denote 

 the parenchymatous islets situated between and dividing the 

 blood-streams (o) ; M, a single seta, mop-like, from the roof of 

 thoracic caviti" ; k, sharp teeth on its shaft ; h, another variety 

 of seta. 



Fi§. 8, A. A single gill of the Lobster, represented as a transparent ob- 

 ject in transverse section : a, section of afferent vessel. The 

 arrows exhibit the division and direction of the afferent blood in 

 its course towards the ultimate branchial tubules (a'"). The 

 ultimate afferent current occupies the axis of each tubule ; 

 a", marks (in section) the great afferent tnmk of the gill, receiving 

 its blood as shown by the arrows from the circumferences of the 

 tubules. B, a single tubule enlarged ; b, afferent vessel, having 

 cribriform walls ; c, c, the efferent stream ; at (c) the capillary 

 system of the tubule is seen. C, the same tubule seen in section. 



D, one of the interbranchial flabella. E, a single mop-like seta 

 from its edge ; /, one from its flat siu-face. 



Fig. 9, A. A single pair of the branchial laminae of the Crab : a^'b, afferent 

 and efferent trunk, connected together by means of the interme- 

 diate branchial lamrnse. These latter are composed of parallel 

 epitheUal plates tied together by means of minute intermediate 

 nodules of fleshy substance. Between these the blood streams in 

 i'mparietal passages. B, horizontal flabellum of the Crab ; C, D, 



E, setae with which it is armed. 



Plate XVIII. 



Fig. 1. Leg, and the projecting free border of the epidermal plates of 

 Talitrus. Intended to express tj'pically the ultimate respiratory 

 structiu-es of all Crustaceans : a, a nodule of fixed parenchj-ma, 

 composed oi slightly refracting oil-cells, nucleated granular cells, 

 and molecules ; — generally such a group is destitute altogether of 

 embracing membrane ; b, the irregular imparietal, angular pass- 

 ages lying between the parenchymatous nodules ; c, the cells of 

 the epidermis, attenuated. 



Figs. 2, 3 & 4. Varieties of epitheUal cells met with in different parts of 

 the Crustacean : a, a cell shown in full, xmder a high power, to 

 bring out its granular character ; b, the same in outline. 



Fig. 5. The parenchj-matous patch a, fig. 1. enlarged, to show its minute 

 structure : a, b, epithelial laminae ; c. represents exactly the 

 pecuhar, low refractive character under which oUy-element exists 

 in the fixed solids of the Crustacea. The whole interior of this 

 patch is permeated by lateral slow-moving currents of blood, 

 diverted from the main stream. 



Fig. 6. A small piece of the wall of the large branchial vessels, showing the 

 hooks (a) on which act the setae of the flabella. 



Fig. /• A portion of the proximal end of a hair, to exhibit the absence of 

 vessels and to show the lacunose to-and-fro character of the 

 blood-movement. 



Fig. 8. The active, vibratory, abdominal palp of the Shrimp. 



Fig. 9. The same viewed transparently : b, c^ epithelial laminse ; e, radia- 



