VII ANNELIDA 153 



Pilidium differs radically from the Trochophore in the fact that its 

 apical plate is cut off with the rest of the larval skin, whereas in the 

 Trochophore larva it persists and forms part of the covering of the 

 adult head. 



REMAINING ANNELIDA 



A great number of other species of Annelida have been worked at, 

 chiefly Polychaeta, but in no single case with anything like the 

 thoroughness with which Woltereck has worked out Polygordius. 

 The Oligochaeta and Hirudinea have no free - swimming larvae. 

 Their eggs are laid enclosed in cocoons along with a milky nutritive 

 secretion, and within these they pursue' their development until they 

 attain the adult form ; so' that, as one might expect, the development 

 is profoundly modified as compared with the Trochophore of Poly- 

 gordius; nevertheless in broad outlines the same general course of 

 development can be discovered. 



POLYCHAETA 



Turning our attention now to the Polychaeta we find that the 

 first important point to be borne in mind is that a true Trochophore 

 larva is found in very few cases. In most cases the rapid develop- 

 ment of the ectodermal descendants of 2d^, which in Polygordius 

 marks the conclusion of the Trochophore stage, begins long before the 

 embryo escapes from the egg membrane ; so that when it does escape 

 it has the form of a post-trochophoral stage with several somites 

 already developed (Fig. 116). 



In a word the development of such worms is, as compared with 

 that of Polygordius, "telescoped." This is true of Nereis among 

 Nereidiformia, Gapitella among Capitelliformia, Arenicola among 

 Scoleciformia, and Amphitrite among Terebelliformia. In other 

 cases, such as Sternaspis, the endoderm consists of large yolky 

 cells, and neither mouth nor anus is developed when the larva 

 begins its free existence. 



It was formerly customary to classify Annelidan larvae by their 

 ciliated bands. Such classification is obsolete, but we shall give 

 the significance of the terms as they are still used by some. 

 Atrochal larvae are those in which there is an apical tuft 

 of cilia and a general ciliated covering, but no prototroch. 

 Such is Sternaspis ; of course they are degenerate forms. Monotro- 

 chal larvae with prototroch only, are the early stages in the typical 

 development of the Trochophore; they become later telotrochal — 

 by the development of the telotroch as described in Polygordius. 

 Mesotrochal larvae, such as that of Capitella, are forms where the 

 metatroch only is developed, the prototroch being undeveloped. 

 Polytrochal larvae are really post-trochophoral stages in which 

 accessory hoops of cilia are developed on the worm's body to assist in 

 locomotion (Fig. 116). 



