ARENICOLA MARINA. 69 



they were distinctly segmented into a series of large cells. The space between the capsule 

 and the contents was more distinct. On the 6th the embryos were ciliated. 



The development of Arenicola Glaparedii has been carefully followed by Ash worth 

 (1901), the larva rotating in the egg at the twenty-eighth hour after fertilisation, and the 

 stomodaeal invagination was present. Twenty-four hours later the telotroch condition 

 was assumed, and one or two red eyes appeared. At the end of the third day the larvae 

 escaped and were 0*25 mm. long. A broad longitudinal band of cilia now occurred on the 

 ventral surface between the anterior and posterior bands. A bristle or two appeared 

 shortly after hatching, but it was four or five days before hooks were seen. They died 

 about a fortnight after hatching, as soon as the yolk was absorbed, for though the mouth 

 was open they did not feed. Post-larval stages were found at the surface in a gelatinous 

 tube. Specimens of Arenicola marina, 3 — 9 mm. long (mounted), have the full complement 

 of gills and all except the first two have become branched. These had reached the end 

 of the pelagic stage, and would soon have settled on the bottom. Ashworth, however, 

 has seen others 6' 5 mm. long without gills, so that there is variety. The smallest specimen 

 found by Ashworth in the sand was 17 mm. in length at the end of June. Young 

 examples are more or less pinkish in colour. 



A small form (Plate LXXXVIIT, fig. 7) was got in the bottom tow-net 11th May 

 inside a loose membranous sheath, which it by-and-by left. A series (nineteen) of long 

 simple bristles project from the surface anteriorly, whilst posteriorly is the narrower 

 tail terminating in the vent. The colour is gamboge-yellow anteriorly and posteriorly, 

 the rest, pale buff. Behind the proboscidian region a section (stomach) of the canal was 

 boldly barred by transverse wrinkles. The posterior region of the gut was devoid of 

 them. No branchiae were present. The transparent tube is secreted by the annelid, and 

 after it left it the body readily adhered to the forceps, a thread of secretion fixing it to 

 the instrument. Minute papillae were present on the posterior (narrow) part of the body, 

 and thus it resembled Arenicola. Two oesophageal caeca occur in front of the stomach. 

 A similar form with one or two additional pairs of bristles was found in the bottom net 

 on the 2nd May. Young stages 1\ mm. long, with nineteen setigerous segments and no 

 gills, and a later stage with eleven pairs of gills, come from Dublin Bay (Southern). 



In a post-larval form (Plate XCIV, fig. 2), about half an inch in length, from extreme 

 low-watermark at St. Andrews in September, 1864, there were four minute eyes anteriorly, 

 and a notch marked the oral aperture. The body was of a general translucent greenish- 

 straw colour, whilst the red blood was most conspicuous in the two sub-intestinal vessels. 

 The ccelomic fluid was pale. The oesophageal caeca were finger-like, and the canal was 

 dilated at their origin. The bristles were slender and slightly curved at the winged 

 point, and the hooks were short and curved. 



In post-larval forms four or five eyes are observed on each side of the prostomium — 

 embedded in the ganglionic layer of the brain (Gamble and Ashworth). They consist of a 

 cup-shaped mass of brown pigment-granules partially enveloping a clear solid-looking 

 body which projects outward. Similar eyes occur in one 25 mm. long. It is difficult 

 to say whether these are present in the adult, from the great quantity of the pigment. 



Gamble and Ashworth l show that the young A. marina may safely be distinguished 

 1 ' Quart. Journ. Micr. Sci./ vol. xliii, n.s., pp. 533-534. 



