574 



THE NATURE BOOK 



centic body and the sidling gait, a scaven- 

 ger of the stream. 



But the shore is the El Dorado of the 

 stone-turning enthusiast. There are few 



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THE HETERONEREIS FORM OF NEREIS. 

 (Magnified five diameters.) 



localities which afford such a wealth of 

 varied life. 



Any casual observer who happened to 

 find a spread of sponge coating the 

 under side of a weed-clustered rock at 

 low-water mark would probably at once 

 suppose the pulpy, greenish-grey mass to 

 be some sort of vegetable growth. There 

 is certainly nothing about its external 

 appearance suggestive of an animal 

 nature. It is a highly irregular growth 



PORCELLANA CRABS AND A MASKED 

 CRAB (PISA). 

 (About half the natural size.) 



full of minute pores, and with raised 

 portions here and there like volcano 

 craters in miniature. If, however, a 

 section through it be examined under 

 the microscope one is surprised to find 

 that what we call the Crumb- of- Bread 



sponge, Halichondria, is in reality a city 

 of waterways, the inhabitants all living 

 very close together. These most minute 

 beings are simple specks of living matter 

 with controlling centres. Those that live 

 along the margins of the waterways have 

 fine hairs, or flagella, which they con- 

 stantly wave to and fro to keep up the 

 water c u r - 

 rents, and 

 thus ensure 

 proper aera- 

 t i o n and 

 regular sup- 

 plies of food. 

 The business 

 of others is to 

 flow out their 

 bodily sub- 

 stance in long 

 " arms," and 

 enclasp pass- 

 ing food par- 

 ticles for the 

 support of the 

 colony ; and 

 some manu- 

 facture won- 

 derful httle 

 needles or 

 spicules of 

 silica for 

 strengtheni n g 

 the walls of 

 their city. The 

 Sponge col- 

 ony shows us 

 not only divi- 

 sion of labour, but also how the higher 

 many-celled animals came to be evolved. 



Various species of a common genus of 

 marine worms, known as Nereis, are 

 abundant under stones in sand and mud 

 between tide-marks. They are usually 

 dark green in colour, and their bodies 

 are much segmented. The photogra]:)h 

 shows one which leads a double life, 

 appearing in two distinct phases during the 

 course of its existence ; first as a Nereis 

 and latterly as a Heteronereis, with one 

 half of the body completely changed. 



A small reddish crab, not generally 

 known, but common enough in the 

 crannies and crevices among stones, espe- 

 cially in mud, is Porccllaua platychcles. 

 His back is flat and so are his large 



CAPRELLA, THE "NO- 

 BODY " CRAB. 

 (Magnified twelve diameters.) 



