BIOLOGY. 



817 



"2. Stomach and viscera enclosed by a fleshy sac, which may be 

 closed or open, at either one or both ends. 



"3. Principal nerve masses consisting of ganglia, which are 

 adjacent to or surround the CESophagus. 



"4. Intestine bending inward, or having an outward flexure. 



** 6. Heart on the outer bend of intestine. 



"Saccata. 



Holozoic, 

 or Typic. 

 Mouth opens 

 anteriorly. 



Phytozoie or 

 Hemitypic. 

 Mouth opens 

 posteriorly. 



{Sac open at 

 anterior end. 

 Sac open at 

 both ends. 



' Sac open at 

 posterior end. 



Sac closed. 



C Cephalopoda. 

 \ Gasteropoda. 



< Lamellibranchiata. 



< Tunicata. 



5 Brachiopoda. 

 \ Polyzoa." 



The paper is also i3ublished entire in the *' American Journal of 

 Science " for July, 1866. 



SILK-PRODUCING SPIDER. 



Dr. Bui-t G. Wilbur has brought to the notice of the scientific 

 world a silk-producing spider, Nephila plumipes, which he believes 

 may become useful in the arts as a source of silk. Those inter- 

 ested in the discovery and habits of this sjiider will find full details 

 in the "Atlantic Monthly," and in the publications of the Ameri- 

 can Association for the Advancement of Science, Boston Society 

 of Natural History, and American Academy of Arts and Sciences. 

 He described the formation of the very large web at the meeting 

 of the American Association. In the first place, the Nej^hila 

 erects her scafiblding, afterwards consumed, and running aljout 

 on that she stretches out her radii, converging to a j^oint four 

 times as near the top of the web as the bottom. Instead of wast- 

 ing time by trying to work round and round like the common 

 spider, impossible to do with the functional centre of her web 

 where she places it, she goes back and forth drawing up each 

 thread at the point where she attaches it to the radius with a sort 

 of loop, which inclines it a little toward the centre. This web is 

 perfectly dry and inelastic, — would never catch a fly. She begins 

 again where the work stopped, and covers the whole web witii a 

 viscid and elastic gum, which arranges itself in drops, according 

 to the attraction of cohesion, along the web. She will not spin a 

 vertical web, but insists on an angle of seventy degrees, and 

 hangs at the functional centre, on the under side of the web. Al- 

 though her vignette shows eight eyes, Dr. Wilder is confident the 

 Nephila is blind to objects, and can only distinguish light from 

 darkness. When a fly is entangled, the spider goes out on a ra- 

 dius to devour it ; but if off her radius she cannot see it, and I'eturns 

 to the centre to shake the web and ascertain from the vibration 

 where its weight drags. He has seen two of these enormous 

 spiders approach each other, entirely unconscious of each other's 

 presence till their legs interlocked without touching. If they 

 touched, ever so slightly, both would turn and run away. The 

 27* 



