74 



Arenicolidae 



In spite of the abundance in innumerable places of the adult 

 worm, and of much searching by many workers, the egg-masses of 

 Arenicola have never been found on the coast of Europe. The egg- 

 masses of only one species A. cristata are known, and they have 

 been recorded only from the eastern shore of the United States, 

 from Wood's Holl to South Carolina. They are huge gelatinous 

 masses, often irregularly cylindrical, three to four feet long and two 

 to four inches in diameter, each of which contains several hundred 

 thousand eggs. The vitellus of the egg has a cinnamon colour, so 

 that in bulk the egg-mass has a reddish-brown tint. 



The early stages of development of two species of Arenicola have 

 been investigated, namely, A. cristata by Profs. E. B. Wilson * and 

 C. M. Child 2 and Dr. E. S. Lillie, 3 and A. pusilla by the writer. 4 



These two species seem to develop along practically identical 

 lines. Within about twenty-four hours after fertilisation, the egg 

 gives rise by spiral cleavage to a blastula, from which a gastrula 

 is formed by growth of the ectoderm cells over the yolk-laden 

 endoderm cells. Shortly afterwards the stomodaeal invagination and 

 the prototroch appear (PL X, Fig. 21), and within three days after 

 fertilisation the paratroch is formed, and one or two eyes are present 

 on the anterior portion of the larva (Fig. 22). About the end of the 

 third day the larva pushes its way out of the vitelline membrane 

 through a thin area which had previously made its appearance. 

 During the following day the first pair of notopodial chaetae is formed 

 (Fig. 23), and in the next two or three days the first crotchets are 

 produced. For the first day or two after hatching the larvae are 

 phototropic and swim near the surface of the water ; but 

 then they begin to settle down, and may be found crawling about 

 the bottom, surrounded by a more or less tubular film of mucus, 

 with which foreign bodies may be entangled. The belts of cilia 

 decrease in size and soon disappear. The alimentary canal is about 

 this time complete from mouth to anus, and is divisible into three 

 regions the anterior and moderately active oesophageal part, the 

 stomach, which still contains a considerable amount of yolk, and the 

 intestine (Fig. 24). The coelom, brain and nerve-cord, and muscle- 

 fibres in the body- wall are clearly recognisable. Shortly after the 

 larvae settle down on the bottom, the pharynx becomes active as 



Stud. Biol. Lab. Johns Hopkins Univ., ii (1882), p. 278. 

 Arch. f. Entwick., ix (1900), p. 587. 

 Mitt. Zool. Stat. Neapel, xvii (1905), p. 341. 

 Liverpool Mar. Biol. Comm., Mem. xi (1904), p. 55. 



