CIRCULATION IN MOLLUSCA. 21 7 



of bloodvessels. In the Bryozoa or Polyzoa, the initiatory 

 class to the mollusca, there exist neither heart, arteries, nor 

 veins, and the nutrient fluid is contained in the great visceral 

 cavity in which the organs of digestion are suspended. In 

 the molluscoid Tunicata there is a heart and a system of 

 bloodvessels in the branchial portion of the body, but there 

 are neither arteries nor veins in the visceral or abdominal 

 portion, in which the circulation is through mazes and 

 lacunas of uncertain direction, and without any definable 

 walls. In the bivalved mollusks the foot is divided into 

 numerous small lacunas or intervisceral interstices for the 

 reception of the inflowing venous blood. On pushing a 

 coloured injection into these intermuscular lacunas, the fluid 

 passes even into the vessels of the branchiae and into the 

 venous canals of the mantle. But in the mantle, as well as 

 in the foot, there do not appear to be any veins properly so 

 called, viz. tubes with distinct parietes to carry the blood 

 from the tissues which this blood is to nourish to the heart 

 or towards the appropriated organs of respiration. It is a 

 system of lacunas only which performs the functions of the 

 network formed by the capillary vessels in the vertebrate 

 animals ; and these lacunas, of almost microscopic size, open 

 into other passages, which, from their disposition, much 

 resemble veins properly so called, but are not so, for they 

 have no walls independent of the adjacent parts. In the 

 typical Gasteropods further progress is made towards a 

 more perfect circulation, for now there are veins as well as 

 arteries, but the former are still partially defective, and are 

 represented, sometimes in the muscular system, and always 

 in the space intervening between the principal viscera and 

 the respiratory organ, by simple lacunas or by the large 

 visceral reservoir, into which the centripetal current flows 

 by open orifices, and whence again it is carried by appro- 

 priate vessels to the branchiae. For example : in the Snail 

 the blood is distributed from the centre throughout every 

 part of the body by the branching arterial system ; and it is 

 returned, partly by proper veins, and partly by mere lacunas 

 or gutters, to the large visceral cavity, into which it is 

 emptied. The blood must consequently be here in contact 



cunes comprises cntre lcs diverges parties solides de ^organization ; d'autrcs 

 fois encore, 1'appareil de la circulation sc perfectionne davantage, car il 

 existe des vcines aussi bien que des arteres dans unc portion plus ou moins 

 grandc du corps ; mais ces veines no suffisent jamais pour completer 1c 

 cercle que le fluide nourricicr doit parcourir, ct la cavite abdominalc ou 

 p6ritoneale jouc tou jours le role d'un reservoir sanguin, aussi bien que 

 d'une cbambre viscerale." — Milne-Edwards in Annates des Scie?ices Na- 

 turelks, ser. 3, iii. 280 (1845). 



