INVERTEBRATE ZOOLOGY. 101 



dorsal blood vessel, and beneath it the nervous cord. The tnbules and glands 

 of the reproductive system will lie symmetrically upon the sides. If the sec- 

 tion be well cut, the relation between the muscular masses and the exo-skele- 

 tal plates will be beautifully shown. The external chitin is difficult to cut 

 and is apt to crack into little pieces, some of which will be carried upon the 

 edge of the knife across the section. This may be obviated by the iise of the 

 following reagent : 



Gum mastic, 

 Absolute alcohol, 

 Ether, 

 Collodion. 

 Take about equal parts of these, put the gum mastic in the alcohol, and allow it to 

 dissolve as much as it will ; then add the other ingredients. 



This is to be applied to the flat section-surface of the paraflftne block, and 

 allowed to dry. The microtome screw is then turned and the section cut off 

 in the usual way. The liquid forms a thin film over the section which holds 

 the pieces of chitin in their natural place. 



72. Preliminanj study of the next three types. These represent three 

 groui^s of minute Crustacea, having an average size of a small pin-head, and 

 constituting the bulk of the material collected with the tow net, whether in 

 marine or fresh water. The type specimens used here are fresh water forms 

 and are collected in the same way as the Turbellaria, Type V. They often 

 develop in quantity in the laboratory aquaria through the chance introduction 

 of a few adult individuals, or from eggs brought in with the mud. Place the 

 material containing these in a shallow crystallizing dish and set over a black 

 surface. Look for small, rajndly moving animals, which may be distinguished 

 by shape and mode of motion. 



1. Somewhat elongated forms, body tapering behind into a tail. The 



females often possess a pair of egg-sacs half as large as the body, 

 depending from the sides of the abdomen. They move by quick, 

 darting motions, often progressing 3-4 inches by a single propulsion. 

 They are thus very difficult of capture and are best taken by a long 

 glass tube, into which a quantity of water may be suddenly drawn. 

 These animals are Copejiods, of which the commonest form is Cy- 

 clops, Type XIII. 



2. Oval forms, somewhat flattened laterallj', i. e. compressed, (cf. de- 



scription of PorceMo, 70, 1.). The body is covered by a pair of lat- 

 eral shells which leave the head free. They are propelled by the sec- 

 ond pair of antennae, which are enormously developed and used as 



