INVERTEBRATE ZOOLOGY. 99 



70. External Anatomy. Use for this specimens killed by simple immer- 

 sion in strong alcohol. Remove tliem. and spread them out on a glass slide 

 npon the stage of a dissecting microscope, allowing them to drj'. The larger 

 external parts are best studied in this way, while the smaller ajipendages, 

 maxillae, etc.. may be mounted temporarily in a drop of water for details. 

 Look over several specimens and distinguish the following : (1) Females car- 

 rj'ing young. These have a brood-sack formed by broad plates attached to 

 the inner side of the first five pairs of legs. In this may be found eggs and 

 young in all stages. Such females possess a median birth-opening between 

 the fifth and sixth pairs of legs, while the lateral genital openings are obso- 

 lete. (2) Females without brood-sack. These lack the appendages upon the 

 legs, also the median birth-opening, but possess lateral genital openings at the 

 bases of the fifth pair of legs. Females change from one of the above forms 

 to the other by a moult. (3) Males. These possess a median organ of copula- 

 tion at the juncture of thorax and abdomen. This folds backwards with the 

 gills, but is generally easily distingx;ished by its darker color. A pair of gen- 

 ital openings lie at its base. 



1. General bodij-form,. The body is depressed, i. e. flattened derso-ven- 

 trally. The thoracic somites are free, and not united into a carapace as in 

 Type XI. On the dorsal side note : — The head ; this consists of the true head 

 of five somites + the first thoracic somite : the long antennae (^second): 

 look carefully on the inner side of the base of these for the very rudimentary 

 first pair: the eyes, a facetted surface on each side. Seven free thoracic so- 

 mites ; the first is concave anteriorlj' and receives the head, the last concave 

 posteriori}', for the small abdomen. The abdomen, six separate pieces, large 

 lateral processes on the second — fourth, the sixth representing the sixth and 

 seventh somites fused, cf. Type XI. 



2. The appendatjes. These may be separated with forceps or sharp needle, 

 the larger ones studied dry and the smaller ones mounted temporarily in 

 water. Those of the thorax and abdomen should be studied first, upon the 

 entire specimen, For the parts of the head, remove this piece by cutting it 

 through from the dorsal side. The parts here are delicate and may be best 

 handled by placing the entire piece in a watch crystal and covering it with 

 water. Make a detailed study of the separate parts, as in the case of the pre- 

 vious Type, and draw each. Refer to the following formula : 



