46 SUMMARY OF CURRENT RESEARCHES RELATING TO 



thick glandular walls and is riclily supplied with blood ; it is, then, 

 the prime seat of the respiratory processes. As it was impossible to 

 examine the small quantity of gas, the question of its real character 

 could not be decided by chemical analysis, but the author concludes 

 that it is oxygen secreted from the mucous membrane of the stomach ; 

 and as the bladders cannot be supposed to have any hydrostatic 

 function, he thinks that they are truly reservoirs of oxygen, which can 

 be called upon at periods of digestion and so on, when the animal is 

 unable to take in a quantity of fresh sea-water to aerate the blood 

 which is passing in such quantities through the walls of its stomach. 

 As to the morphological significance of these appendages which 

 have already been shown to be diverticula of the fore-stomach, we find 

 them to be, in all probability, a product of the endoderm. The 

 variations in its development which are to be seen among the Syllidea, 

 with the general characters of Syllis, and the absence of any special 

 enteric vascular system in Tyrrhena, lead to the conclusion that the 

 atrophy of the bladders in some of the Syllidea is due to the develop- 

 ment of the dermal mode of respiration. In all Annelids in which 

 gills are wanting, and these. gills are no peculiar developments, an 

 enteric mode of respiration would appear to obtain. We may, in 

 conclusion, suppose that, in the ancestors of the Fishes of the present 

 day enteric respii-ation existed (as it does to this day in Cobitis) ; in 

 some this mode led to the formation of a reservoir, which under 

 hydrostatic influences took on the function of a hydrostatic organ. 

 At the present day we see that a fish uses up all the air in its air- 

 bladder before it is suffocated, and even that a piJmonate Vertebrate 

 uses its lungs, in water, as a hydrostatic organ. 



Development of Polygordius and Saccocirrus.* — W. Eepiachoff 

 finds that in both these lowly Cheetopods the cleavage of the ova is 

 total ; that after eight segments have become developed the embryonal 

 cells begin to develope one after another ; the gastrula is formed by 

 invagination ; while the mesoblast of Polygordius appears to be de- 

 veloped from the hypoblast, in Saccocirrus " primitive mesodermal 

 cells " are to be found within the cleavage cavity. Even during the 

 blastula-stage the embryos of Polygordius begin to swim about by 

 means of very fine cilia ; after the closure of the blastopore the larva 

 becomes more vermiform ; the now-closed anterior end remains, how- 

 ever, for some time distinctly swollen out. Movable hairs appear at 

 scattered points on the surface of the larva, which give to the creature 

 something of the appearance of a larva of Sagitta ; later on, two cirri 

 become developed at the anterior end, but this species of Polygordius 

 (P. Jiavocapitatus) never passes through the stage of the Lovenian 

 larva. 



Termination of Nerves in the Voluntary Muscles of the 

 Leech.'l' — A. Hansen states that the nerves divide and subdivide 

 without forming anastomoses, and lose themselves in the muscles 

 without our being able to discover their terminations ; in only one 



* Zool. Anzeig., iv. (1881) pp. 518-20. 

 t Arch, de Biol., ii. (1881) pp. 342-4. 



