ANNELIDA 2'?5 



[directly compared to the perianal ciliated band of other An- 

 nelid larvae, for it is not, like that, situated at the posterior 



[end, but a number of segments make their appearance be- 

 tween it and the hind end. The anus in these lartaeJ is 

 placed dorsally, for a pointed prolongation is formed pos- 



[teriorly on the ventral surface, a condition which also recurs 

 in polytrochal larvae (corap. Fig. 127). 



Noteworthy is the tuft of cilia at the anterior end which we met with 

 in the atrochal and mesotrochal larva, and which is also recalled by the 

 stout cilia found at the head end of many telotrochal and polytrochal 

 larvffi. Such a ciliation of the apical area occurs also in Turbellarian, 

 Nemertean, and Molluscan larva, and has perhaps a higher significance 

 than that of a merely secondary acquisition, connected with the larval 

 mode of life only. 



Apparently aberrant larval forms, such as those of Mitraria 

 (Fig. 124 A, B), are referable to the Trochophore. Mitraria 

 would, therefore, have to be classed with the Monotroch(E, in 

 which a preoral, but not a perianal, band of cilia is de- 

 veloped (comp. the larva of Psygmobranchus, shown in Fig. 

 128, p. 280). Monotrochce and Telotrochce cannot be separated 

 from each other, inasmuch as in the beginning the larvae 

 frequently possess only a preoral band of cilia, are therefore 

 monotrochal, whereas later a perianal ciliated band, which 

 gives them the character of Telotrochce^ is developed on them. 



Mitraria, the Annelid larva discovered by Joh. Mullee, and subse- 

 quently more thoroughly studied by Metschnikoff, can easily be recog- 

 nized in its young stages as a Trochophore, with a well- developed bell, but 

 much-reduced posterior portion (Fig. 124 A). As a result of this, the 

 anus and mouth are brought close together. The ciliated band lies in 

 front of the mouth. Later the posterior part of the body grows out 

 more, and the ciliated band, which acquires many outfoldings, there- 

 fore comes to lie more anteriorly (Fig. 124 B). In this figure the 

 beginning of the worm, which is gradually developed out of the 

 Mitraria, can be easily recognized. On the lower area, surrounded by 

 the ciliated band, two lateral protuberances, which bear long cilia, can 

 be recognized in the young larva. In the older larva they are seen 

 lying dorsally. The metamorphosis of the larva into the tubicolous 

 worm is due to the vigorous growth of the segmented posterior portion 

 and the degeneration of the chief part of the Mitraria, together with 

 its lobes and setiferous papillae. Thereupon the larva sinks to the bottom, 

 secretes the tube, and becomes attached. 



