ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 45 



When a detailed examination is made of M. sicula it is found that 

 the enteric tract may be divided into (a) a proboscis with an oeso- 

 phagus, (h) the proper digestive gut, and (c) an intermediate fore- 

 stomach to which the bladders are attached. The first and last 

 portions of the gut (a and h) differ not only in external appearance, 

 but also in structure ; while in the former the epithelium is feebly 

 developed, in the latter it gives rise to a well-developed mucous 

 membrane ; the constituent cells are greatly elongated and are ciliated 

 at their free extremity ; it is also richly supplied with blood-vessels 

 which, as in the clitellum of the Earth-worm and the epithelium of 

 the Leech (Lankester), interpenetrate between the cells ; this rare and 

 remarkable arrangement would appear to be explained by the fact 

 that in the Hesione (as in Syllis) the gills are absent. 



The fore-stomach has very thin walls and when contracted is 

 hardly of the length of a somite ; it is, however, capable of great 

 extension ; in its structure it is intermediate between the CESophagus 

 and the gut proper ; for, while it resembles the former in the characters 

 of its epithelium, it has the musculature of the latter. At its side the 

 two bladders open. Till they approach their orifice these bodies have 

 a ventral position, they appear to be easily contractile and of great 

 extensibility. When full they form saccular reservoirs ; when empty 

 cylindrical tubes, which gradually diminish in diameter towards their 

 blind end. Their orifices are wide, but there is a means by which food 

 is prevented from entering them, and the valvular arrangement is such 

 that, the mouth or anus being closed, gas or water enters them where 

 the gut contracts, and water or gas passes from them into the gut when 

 they contract. In general structure they resemble the fore-stomach, 

 of which therefore they may be regarded as diverticula. 



On examiniag the characters of the blood-vascular system we see a 

 dorsal double trunk, a ventral single, and two lateral ones ; the two 

 former, by numerous anastomoses, carry venous blood to the walls of 

 the stomach, whence it passes to the lateral trunks ; these are of con- 

 siderable size and contract rhythmically and supply the greater part 

 of the body by the thirteen arteries which are given off from them to 

 as many somites. The ventral and the lateral vessels are also in 

 direct connection by several anastomoses, and each artery is likewise 

 in communication with the ventral enteric vessel. In other words, 

 the greater quantity of blood is brought into connection with the 

 intestine. 



Other Hesionids and the Syllidea are then described; after which 

 the author passes to a consideration of the function of the swim- 

 bladders ; these were never found to contain food or to give rise to 

 any secretion ; they contained nothing but a varying amount of clear 

 fluid and gases, both of which could be driven into the stomach, or 

 vice versa. The fluid is sea-water, taken in from without, and this 

 water appears to be taken in for respiratory purposes. As to the 

 " air," experiment first of all showed Dr. Eisig that it was not atmo- 

 spheric air, and the question whether it was secreted in the animal 

 itself was examined, after the following considerations; the air- 

 bladders are thin- walled, elastic, and without blood-vessels; the gut has 



