130 



INVERTEBRATA 



CHAP. 



glassy transparency. The preparations made in this way, however, 

 are not permanent, but they last long enough to enable good draw- 

 ings to be made. Other authors make permanent preparations by 

 preserving the eggs in "Eisig's mixture," i.e. 3 parts of saturated 

 aqueous solution of corrosive sublimate and 1 part of glacial acetic 

 acid, staining with haematoxylin and trusting, after dehydration by 

 alcohol, to oil of cloves to clear them sufficiently to allow of complete 

 examination. 



In order to examine the eggs from all sides Wilson rolls them 

 about on the slide by moving the coverslip, which he supports on feet 



made of a mixture of 

 beeswax and vaseline, the 

 proper height of which 

 can be ascertained by 

 trial. Other workers 

 attain the same end by 

 introducing between 

 slide and coverslip a piece 

 of thin capillary glass rod 

 or tube, drawn out to 

 the requisite degree of 

 tenuity. 



When the segmenta- 

 tion is completed the 

 embryo issues from the 

 egg membrane and com- 

 mences to lead a free 

 life as a larva. The form 

 of this larva resembles 

 in broad outline the form 

 of the Pilidium. Like 

 it, it possesses an apical 

 plate with a tuft of long 

 cilia and a prototrochal 

 girdle, and it is called a 

 Trochophore. 



The Trochophore 

 differs from the Pilidium 



int 



AN. 



mt.tr 



FIG. 100. The Trochophore Larva of Polygordius, 

 viewed from the side. (After Woltereck. ) 



A.N, archinephridium ; ap, apical plate ; int, intestine ; 

 mt.tr, metatroch ; o, mouth; p.tr, prototroch; st, stomach; 

 t.tr, tetotrc^h. 



in possessing an intestine 

 terminating in an anus, 

 and in having a post- 



trochal region of the body which projects behind the prototroch, 

 instead of having only a concave surface in this position such as is 

 found in the Pilidium. By the gradual growth and elongation of the 

 post-trochal region, the body of the worm is formed. 



In the case of the Serpulid Pomatoceros the eggs and sperm are 

 easily obtained by simply extracting the animals from their tubes and 

 placing them in clean sea-water. If the genital cells are ripe they 



