ANNELIDA 281 



the forms mentioned (Exogone) is said to pass through a 

 stage of development which, according to the description of 

 ViGUiER, strongly resembles the " larva? " of the Oligoch^eta. 



2. Development without Free-swimming Larva 



(^OligocJiseta). 



The Oligochastalay their previously fertilized eggs in firm 

 cocoons, consisting of a chitin-like substance. The cocoons 

 vary greatly in shape in the different genera, and, according 

 to the life-habit of the worm, are found either in the earth 

 or attached to aquatic plants. The slender, spindle-shaped 

 cocoons of Criodrilus attain a considerable length (as much 

 as 5 cm.). In the Lumbricidce they are I'ounded or ovate, 

 and of different sizes in one and the same species, being 

 about as large as a pea or a bean. Correspondingly the 

 number of eggs which they contain is also variable. Some- 

 times only a very few eggs are found, while in other cases 

 the number may i-each as many as twenty or thirty. Usually 

 not all of these eggs dev^elop, but, as appears, some of them 

 develop at the cost of the others. Ordinarily the eggs float 

 in an albuminous mass. Their development is different 

 according as they contain a small amount of food-yolk 

 (Lunibricus, Criudrilus), or possess abundant yolk (BIiijh- 

 chelynis, Tubife.v^. Cleavage is always unequal, but in the 

 first case an invaginate gastrula is formed, while in the 

 second an epibolic gastrula occurs. 



Cleavage and the formation of the germ-layers in the Oligochseta have 

 been thoroughly studied by various investigators (Kowalevsky, No. 27 ; 

 Hatschek, No. 18; Kleinenberg, No. 24; Vejdovsky, No. 45). In 

 Lumhricus a blastula is formed which is thicker on one side, and which 

 may be flattened so that the cleavage cavity is small ; and the gastrula, 

 which soon arises by invagination, is also at first rather flat (Kowalevsky, 

 Fig. 130 A). These characters are less marked in the case of Lumhricus 

 trapezoides (Fig. 129 A), in which occurs the peculiar phenomenon of the 

 division of the embryo in the gastrula stage, producing in this way two 

 embryos, which, separated from each other, develop further. Fig. 129 

 A represents such a stage of division of an embryo, and shows the two 

 embryos (which are in the same stage of development) only slightly 

 united. 



