108 INVERTEBRATE ZOOLOGY. 



80. Development. Cypris develops by a Naiiplias, which is a fine illus- 

 tration of caenogenetic modification, or that modification by which an impor- 

 tant modern character develops during the repetition of an ancestral stage. 

 Here the Naujilias is provided with the characteristic bivalve shell. Search 

 at the bottom of an aqnarinin filled with Cypris. and examine the smallest 

 forms. The shells are transparent and some of them may be seen to possess 

 but three pairs of appendages, the number characteristic of Naupliae. 



8 I . Comparison with barnacle larvae. The common barnacles, which 

 cover the rocks and all submerged objects at the sea-coasts, are modified 

 Crustacea, similar to Ostracods, which develop as free-swimming Naupliae, 

 and afterwards become sessile in an inverted position. When sessile, the 

 shells develop calcareous plates, and the appendages grow into delicate ten- 

 dril-like extremities, suitable to direct to the mouth a current of sea water, 

 containing nutritious material. The Nauplias larva is similar to that of the 

 Ostracods, and a later stage, the so-called " Cypris" stage, is similar to the 

 adult. Barnacles may thus be considered modified Ostracods. The young 

 stages are commonly found in marine tow, which should be diluted and exam- 

 inetl as in the case of Medusoids. The Naupliae of marine Copepods, simi- 

 lar to Cyclops, are also to be expected, but these are distinguished by the ab- 

 sence of lateral shells. 



Type XVI.— A Spider (almost any form will do). 



82. External characteristics. Spiders may be killed by chloroform, ben- 

 zine, or by immersion in alcohol. All spiders are venomous, but only the 

 large ones have mandibles powerful enough to penetrate the thick ciTticle at 

 the finger tips. They can bite onlj' what is beneath them, and may thus be 

 firmly grasped from above with the thumb and finger. They may often be 

 induced to drop into a wide mouthed bottle filled with alcohol, by holding it 

 beneath them, and poking them carefully with a stick. For external parts, 

 alcoholic specimens may be used, but large fresh specimens must be selected 

 for dissection. Investigate the following external characteristics : — 



(a) Body. Two divisions, a cephalo-thorax and an abdomen. Anterior 

 end s(xuare with abriipt anterior edge, upon which are situated sim- 

 ple lenticular ej^es, eight in most spiders. The arrangement and rel- 

 ative size of these is an important systematic distinction. Compare 

 and draw several different forms. The waist is attenuated and the 

 abdomen is generally so convex dorsally that it laps over the poste- 



