REPRODUCTION AND GROWTH 267 



of the sedentary Polychasts : Pomatoceros triqueter and 

 Hydroides norvegica attain nearly full size in about four 

 months and are then sexually mature ; Polymnia, Daschyojie, 

 and many others have attained a good size in considerably 

 less than a year ; Serpula verrmcularis was found to be 

 sexually mature at not more than ten months old ; Sabella 

 pavonina formed a tube i2*6 cms. long in less than thirty-one 

 weeks, and at this age contained well- developed but not 

 quite mature ova. 



Crustacea. — Perhaps the most characteristic feature of 

 the breeding of shore Crustacea is the long period of incuba- 

 tion, during which the eggs are carried about by the female. 

 In the Copepods, the commonest shore representatives of 

 which belong to the Cyclopidae and Harpactidae, the eggs 

 are carried in sacs, usually paired and placed at the sides 

 of the abdomen. These sacs are easily seen if females of 

 the above-mentioned families are collected from among the 

 algae of shore pools and examined under the microscope. 

 The habit of carrying the eggs about is no doubt an adapta- 

 tion to shore conditions, and is also seen in fresh- water 

 species of Copepods. 



In the higher Crustacea {i.e. Decapoda) the eggs are 

 usually attached by a sticky material to the swimmerets of 

 the female, but in the Stomatopoda they are carried by the 

 female in a temporary chamber formed by the apposition 

 of the large maxillipeds. 



The sexes are separate, except in sessile and parasitic 

 forms, which follow the usual rule in such cases and are 

 hermaphrodite. The chief types of larvae are the Nauplius 

 and Cypris, characteristic of the Entomostraca, and usually 

 associated in the same life-history ; and the Zooea and 

 Megalopa, confined to the Malacostraca, also consecutive 

 stages of a single life-history. The larval stage, however, 

 may be considerably abbreviated, when the young animal, 

 which differs from the adult only in points of detail, is known 

 as a Mysis {e.g. lobster). As a curious modification of the 

 Mysis we have the Phyllosoma larva of the spiny crayfish 

 (see Fig. 17). 



