58 W. BALDWIN SPENCER. 



pressed against the outer wall of the female opening. There 

 is thus produced both an internally lying space in the tube on 

 either side in the male down which the cirrus can pass from 

 its sac, and also a median space in the female opening into 

 which the rod-like processes direct the cirri. The projection 

 of the latter is caused by two contrivances; (1) the muscles in 

 the wall of the cirrus-sac, and (2) the sudden ejection of a 

 mass of spermatozoa by the ejaculatory tubes with their 

 remarkably strong muscular walls. The whole arrangement 

 may, in fact, be regarded as consisting of two more or less 

 flexible tubes (the cirri), attached at one end to the wall of a 

 compressible chamber, in which when at rest they lie coiled up 

 with their distal end free. A combined sudden ejection of 

 material through the tubes and compression of the walls of 

 the chamber will inevitably lead to the ejection of the tubes 

 themselves, which then naturally pass along the cavity espe- 

 cially opened for them, and formed by the male genital openings 

 and the vagina of the female. 



It is, however, difficult to understand how the cirri when 

 once put out are ever drawn in again. Possibly they never 

 are, but are simply broken oiF, and then gradually by muscular 

 action of the vagina ejected from the female. This might 

 account for the presence of only an extremely short cirrus in 

 a form such as the one represented in figs. 62 and 66, where 

 the animal was otherwise quite mature. 



Copulation in P. teretiusculum does not take place till 

 the female is of considerable size, as in form 12 mm. or even 

 more in length there is no trace of spermatozoa anywhere in 

 either the vagina or receptacula. In the snake's lung also 

 each individual has apparently its fixed place, with its head 

 deeply sunken into the tissue of the lung, to which it is 

 attached by its hooks, the deep depression corresponding in 

 configuration with its head end. 



On each side the cavities of the dilator rod-sac and the 

 cirrus-sac open into a common tube (fig. 11, Co.), the walls of 

 which are formed of (1) an internal layer of cells with a dis- 

 tinct cuticle, (2) a layer of longitudinal muscles-fibres, and 



