366 DB W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



as transparent objects. It is almost unnecessary to contradict the last statement, 

 since small specimens of most of the species are more or less translucent. This 

 author also maintains that the group agrees in the structure of its generative 

 organs with the type of the lateral ovarian pouches of the Hirudinei, differing 

 from the latter, however, in having the sexes separate. MM. Van Beneden 

 and Keferstein give a correct account of the position of the ova and sperm-sacs 

 in the body of the species examined by them; but the term "biliary caeca" 

 used by the former is objectionable, as tending to confound the generative and 

 digestive systems. 



M. de Quatrefages makes no mention as to how the ova are extruded, 

 though he points out that (Ersted and Duges were wrong in averring that they 

 escaped through the walls of the body. (Ersted's observation, however, is cor- 

 rect, as subsequently proved by MM. Beneden and Keferstein. Frey and 

 Leuckart erroneously conjectured that the ripe ova were shed from the posterior 

 end of the body, " as in Arenicola." 



The unimpregnated ova in 0. alba (Plate VIII. fig. 8) are pure white, and mea- 

 sure about ^d of an inch in diameter, the pale spot just before deposition being 

 about ^-gth of an inch. The ovum has two coats — an external hyaline investment 

 (a), which becomes considerably firmer after extrusion, and an inner membranous 

 sheath (6) of greater delicacy enveloping the vitellus (c). With the exception of 

 the pale spot the ovum is uniformly granular, the granules on gaining freedom 

 showing very active molecular motion in the surrounding water. At a particular 

 point there is a very distinct process (micropyle?) (d), as if from the remains 

 of a tube that led through the outer coat. In a few hours after deposition and 

 impregnation the pale spot disappears, the yolk divides into two masses, and 

 shortly afterwards into four. On the second day they are almost all in the 

 mulberry-stage. In seven or eight days the contained embryo is observed to 

 revolve within the capsule by aid of its cilia, and the majority are extruded from 

 the 12th to the 14th day. The young animal is furnished with two eyes before 

 bursting the egg (Plate VIII. fig. 11), and the coarse granular matter and globules 

 of the digestive tract are apparent. In such a condition the wall of the ovum is 

 readily ruptured, and in several instances the posterior end of the animal emerged 

 first. No sooner did the young get over their labours of extrusion than they 

 glided rapidly off, head first, in a manner that showed no training was necessary 

 to enable them to progress. Probably the action of the cilia may have some 

 influence in determining their course. In these young animals, which are just 

 visible to the naked eye as minute specks, the proboscis is marked by a paler 

 space (Plate IX. fig. 1), that has on each side of it a dense mass of the granules 

 of the digestive canal. To the outside of the latter are two pale stripes, broader 

 in front, caused by the nervous ganglia and trunks. Two longer cilia mark the 

 posterior end. A further stage of development (after an interval of about eight 



