NOTES 469 



10 This digestive cavity is really homologous to the proboscis of the 

 jellyfish, turned in. It is lined with ectoderm. The "body cavity" is 

 not really such, but is homologous to the digestive sac of the hydra. 



11 Among the exceptions are Tubipora, which have eight tentacles and 

 no septa, and the extinct Cyathophylla, whose septa are eight or more. 



12 The longer septa (called primary) are the older; the shorter, sec- 

 ondary ones are developed afterward. As a rule, sclerodermic corals are 

 calcareous, and a section is starlike; the sclerobasic are horny and solid. 

 The latter are higher in rank. 



13 The most important genera are Terebratula, Rhynchonella, Discina, 

 Lingula, Orthis, Spirifera, and Productus. The first four have represen- 

 tatives in existing seas. Most naturalists now admit their affinity to the 

 worms, some still keep them in the branch Mollusca, while others include 

 them in the separate branch Molluscoida. 



14 Some starfishes {Solaster} have twelve rays. In all echinoderms, 

 probably, sea water is freely admitted into the body cavity around the 

 viscera. 



15 The shell is not strictly external, like the crust of a lobster, but is 

 covered by the external skin. 



16 Six hundred pieces have been counted in the shell alone, and twelve 

 hundred spines. The feet number about eighteen hundred. They can 

 be protruded beyond the longest spines. 



. 17 Certain crabs live on dry land, but they manage to keep their gills 

 wet. 



18 The student should remember that this threefold division is not 

 equivalent to the like division of a vertebrate body. 



19 Each ring (called somite) is divisible into two arcs, a dorsal and 

 ventral. 



20 The eye stalks were formerly considered to be appendages, but are 

 no longer so regarded. 



21 These parts do not correspond to the parts so named in human 

 anatomy. See also pp. 371, 372. 



22 The four pairs of legs in arachnids answer to the third pair of max- 

 illge and the three pairs of maxillipedes in the lobster. The great claws of 

 scorpions and the pedipalpi of spiders correspond to the first maxillae of 

 the lobster. 



23 Compare the single thread of the silkworm and other caterpillars. 



24 The common spider, Epeira, which constructs with almost geometri- 

 cal precision its net of spirals and radiating threads, will finish one in forty 

 minutes, and just as regularly if confined in a perfectly dark place. 



25 There are some exceptions : the oyster is unequivalved, and the pecten 

 equilateral. 



26 The chief impressions left on the shell are those made by the muscles 

 the dark spots called "eyes" by oystermen ; the pallial line made by 



