340 ASTACOCKOTONj A NEW TYPE OF ACAEID, 



slightly elliptical, organs of about .04 mm. diameter in the male and about .05 to 

 .1 mm. in the female. These organs (fig. 19, gu) have every appearance of be- 

 ing glands; they are composed of distinctly nucleated closely-packed elongated 

 cells of about .01 mm. diameter and in section exhibit similar cellulation all 

 through; they ai-e quite solid without lumen, but I have not been able to trace 

 any duct from them. I thought at one time that they probably discharged into 

 the posterior part of the tubular salivary glands, but after careful investigation 

 I am not, up to the present, able to state that this is the case, although the two 

 organs are in tolerably close juxtaposition; and the function of the glands there- 

 fore remains uncertain to me" (1895, p. 197). 



A pair of organs which are almost certainly homologous with those described 

 occur in the female Astacoeroton. They are a pair of rounded organs nearly .2 

 mm. in diameter, ventro-laterally situated between the mesenteric diverticulum 

 and the uterus, embedded in the posterior wall of the latter. Each is a solid 

 mass of polyhedral cells averaging about .02 mm. in diameter, without lumen and 

 without duct. 'Ihere is certainly no connection with the tubular salivary gland, 

 the bend of the loop of which is definitely anterior. There is a very close as- 

 sociation between the cells of this gland and those of the uterine epithelium, 

 but if the former organ occurs, as Michael states it does in Thyas, in both sexes, 

 its function is not easy to determine. In the only male Astacoeroton (immature) 

 of which I have sections it was not seen. 



11. Male. 



I am not in a position to give a full account of the male since I have only a 

 single specimen, and that is in an immature condition. I have never found a 

 male attached to tlie gills — the few I have seen being free in the gill-cavity. Their 

 apparent scarcity may be partly due to their freedom. The specimen referred to 

 was very small, not more than a millimetre in length. The appendages are not 

 distinguishable from those of the female. T'he presence of a pair of eyes may be 

 peculiar to the male ; but early stages of the female may possess them. ' The 

 chief — if not the only — external difference distinguishing the male is in the re- 

 productive aperture, which is a comparatively short slit situated relatively far 

 forward, about the middle of the ventral surface. 



Sections of this specimen show that the reproductive apparatus is still in a 

 very rudimentary condition. TJie testes are a pair of sacs with distinct empty 

 lumina. These open in front into a narrow median passage (vas deferens) lead- 

 ing to the genital aperture. Behind they unite together in the middle line, their 

 cavities communicating. The two testes and their connections thus form a kind 

 of ring. The walls of the testes are composed of a mass of minute cells of 

 uniform character. Accessory glands are recognisable as a group of cells about 

 the median vas deferens and its external aperture. In the mature condition it 

 is obvious that the lumina of the testes will act as vesiculae seminales, and their 

 continuation to the median vas deferens as the lateral vasa deferentia. 



12. Conclusion. 



It appears to be almost certain that the nearest relatives of Astacoeroton 

 are to be looked for among the Hydrachnida; no mite not adapted to aquatic life 

 could conceivably have given origin to a permanent external parasite of an 

 aquatic animal. 'The marine derivation of such a hypothetical non-parasitic or 

 partly parasitic ancestor is not necessarily excluded. Living alongside Astacoero- 

 ton in the gill-cavity of the crayfish is Stratiodrilus, whose only known relative 



