71 8 MB. T.. A. BORKADAILE ON THR SPAWN [Dec. 1 7. 



first traces of a third seta-sac with a partly-formed compound seta 

 were observed. The second seta-sac lias now three compound 

 setae. The body is much elongated, and the unciliated region 

 preponderates greatly over the ciliated band, which lies wholly 

 in front of the mouth. The apical tuft is lost. Rudimentary 

 parapodia bear the two anterior pairs of seta-bundles. At the 

 hind end a small blunt knob appears on each side of the anus. 

 The pharynx is elongating, and a faint double line runs down 

 its long axis, diverging into two at the anterior end and thus 

 forming a Y. This is the rudiment of the lower jaws (Unter- 

 hiefer, inferior maxillary plates). 



8. Sixty hours after hatching (PI. XXXIX. fig. 9) the embryo 

 has elongated until its form may be said to be definitely worm- 

 like. There are now three seta-bundles raised on low parapodial 

 eminences. The first has two compound and two simple seta?, 

 the second three compound, the third two compound. Behind 

 these may be seen rudiments of a fourth pair of seta-sacs. The 

 anal processes are larger but still rounded. All the cilia have 

 disappeared. The mid- gut has the form of an elongated isosceles 

 triangle, with the apex, which is truncated, situate at the anus, 

 and the base rounded and placed just behind the eyes ; it is still 

 very yolky. In the fore gut (pharynx) the lower jaws are better 

 formed, and their diverging limbs bear each three blunt teeth. 

 Rudiments of the large toothed plates of the upper jaws have 

 appeared. But the most important feature of this stage is the 

 beginning of the segmentation of the region in front of the first 

 pair of parapodia. In this region, which has hitherto formed 

 a large unsegmented " head," there now appears a low bulging 

 of the body-wall on each side just before the first parapodium. 

 Thus the segment bearing the latter becomes the second after the 

 head, and eventually, as we shall see, the third. The new segment 

 is unprovided with setae. It is interesting to notice that the two 

 segments which thus arise so differently from the rest are, in the 

 adult Marphysa, as in various other Polychaetes, distinguished 

 by not having parapodia. The exact meaning of the process is 

 doubtful ; but it must be of camogenetic origin, for Salensky (7) 

 has shown that this portion of the body belongs to the trunk, not 

 to the head. 



9. Twenty-four hours later (i.e., eighty-four hours after hatching) 

 a second segment without setae has appeared in front of the first, so 

 that the body now shows a rounded head followed by two seg- 

 ments without setae, three with setae, and a hinder portion from 

 which the fourth seta-beai'ing segment has not yet differentiated 

 itself. All the segments are better marked off from one another 

 than on the preceding day, and the parapodia stand out more 

 sharply from the body. The anal processes are longer, taper 

 slightly towards the tip, and curve outwards. 



10. In the course of the next twenty-four hours the bodv 

 elongates somewhat, the fourth pair of parapodia appear with two 

 compound setae, and the food-yolk grows considerablv less. 



11. The oldest larvae examined by me (PI. XXXIX. fig. 10) 



