300 Dr. T. Williams on the Mechanism of Aquatic 



muscles exist in these extreme situations. No separate vascular 

 membranes bound the peripheric blood-currents; therefore no 

 " capillaries '' exist in the Crustacea. The fact then is now 

 susceptible of general expression, that in the articulated animals, 

 most certainly in Insects and Crustacea, the peripheral circula- 

 tion in consequence of the presence of a firm unbending epider- 

 mal skeleton, cannnot by mechanical possibility be any other 

 than it is, namely a profusion of irregularly subdivided streams, 

 traversing angularly bounded passages in fixed non-contractile 

 inflexible solids. An exception to this axiom may exist in the 

 example of some of the internal organs — probably in the musculo- 

 glandular walls of the alimentary canal, certainly not in the liver 

 of the Crustacea, as will be afterwards shown. 



Let now these general anatomical facts be applied to the ana- 

 lysis of the branchial organs in their several varieties in this 

 class, or to the mechanism of the respiratory act, where there 

 exists to this end no separate provision. 



The araneiform Crustaceans are furnished with no separate 

 respiratory organs. Almost every English systematic writer de- 

 scribes the Pycnogonidse as destitute of a true circulating 

 system*. This is an error. In Pycnogonum the existence of a 

 dorsal vessel lying on the dorsal aspect of the stomach may be 

 readily demonstrated. The blood follows the csecal diverticula 

 of the stomach into the legs ; it returns by separate channels 

 along- the ventral aspect of the cseca into an auricular division of 

 the heart. The peripheral blood-currents do not subdivide. The 

 solids are not permeated by subdivided capillary currents. Every- 

 thing beyond the main stream is cyclosis — that is, non-corpus- 

 cular fluid passes by endosmose from cell to cell. This extra- 

 vascular movement of fluid plays a part in the nutrition of the 

 solid structures of Invertebrata, of which the frequency and the 

 extent are by no means yet rightly estimated by the physiologist. 

 The floating corpuscles of the blood never pass beyond the walls 

 of the proper vessels : they never reappear de novo in the fluid 

 beyond the vessels. In the latter region the fixed cells impress 

 upon the blood required changes. But this extra-vascular fluid 

 after leaving the vessel, may unquestionably undergo the process of 

 aeration. This is exemplified in the Pycnogonidse, in which the 

 blood-current is so little subdivided. In this group the floating 

 corpuscles are relatively to the size of the animal very large. 

 They conform to the crustacean type; they are granular and 

 nucleated, suspended in a clear, colourless fluid. They move in 



* " In one of the most degraded forms of the class, we revert to the sim- 

 plest possible type of the circulating apparatus ; even the dorsal vessel, 

 which is so chai'acteristic of the Articulata, being apparently deficient in the 

 Pycnogonidae." — Dr. Carpenter's Principles, &c., p. 695. 



