AVIAN CESTODES. 867 



a circular cavity projecting beyond the rest of the genital cloaca. 

 The general shape is suggestive of a silk hat, if the brim of 

 the hat were hollow. From the centre of the lumen of the 

 genital cloaca a slender tube, which from its structure appears to 

 be morphologically a part of the genital cloaca, leads to the 

 junction of the male and female ducts. This is shown in text- 

 fig. 144. It should be noted that the muscular layer of the 

 cloaca genitalis is covered externally by a layer of subcuticular 

 cells continuous with those which underlie the body cuticle. It 

 is, therefore, perhaps to be presumed that the muscular layer in 

 question is a local thickening of the delicate layer of fibres which 

 underlies the body cuticle. Occasionally I have observed the 

 cloaca genitalis to be slightly protinided ; in such cases the cavity 

 was more cup-like than cylindrical owing, of course, to the gaping 

 of the external oiifice. 



The sperm-duct and cirrus-sac, as already mentioned, pass 

 obliquely forwards in the mature segments and are parallel to the 

 vagina, which follows an identical course. The cirrus-sac in this 

 worm is large and directed in a straight line towards its opening 

 into a chamber in common with the vagina. This latter chamber 

 has nothing to do with the terminal cloaca genitalis from which 

 it is sharply marked off by the muscular walls of the cloaca, 

 which have a narrow tube of intercommunication, doubtless 

 capable of being widened. In all the individuals which I ex- 

 amined the cirrus of the fully mature progiottids was largely 

 protruded from the cirrus-sac, but not through the cloaca 

 genitalis to the exterior of the body. I found the cirrus, in fact, 

 to be invariably inserted into the neighbouring vagina, which 

 latter was as invariably filled with spermatozoa. There is, of 

 course, nothing new in this record of self-fertilization, which is 

 well known to occur among Cestodes, and has been recorded by 

 van Beneden, Leuckart, and others. A large number of instances 

 are given in Bronn's ' Klassen und Ordnungen des Thierreichs ' 

 by Prof. Max Braun *. I have not, however, noticed it myself 

 in the considerable number of species which I have examined, 

 excepting in the present species. This auto-copulation is there- 

 fore far from being universal in occurrence. It appears to me, 

 furthermore, that the structure of the efferent apparatus in the 

 Oestode, which forms the subject of the present communication, 

 may at times necessitate this auto-copulation. A closure of the 

 cloaca genitalis coinciding with the eversion and protrusion of the 

 cirrus would force the latter into the vagina, which is widened at 

 its extremity, and does not project into the common chamber 

 into which both efferent ducts open f. The passage for the cirrus 

 is therefore not in any way hampered. 



The cirrus-sac has thickened walls, as in so many species, and 



* Bd. iv. Abth. 1, p. 1462. 



t V. Janicki in describing Schisotcenia haymanni (Zeitschr. wiss. Zool. Ixxxi. 

 1906, p. 585), where there is also a very deep cloaca genitalis, comments on the 

 mechanical necessity for anto-copulation on rather different grounds. 



