250 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



cultrifera and of Psygmobranchits protensus) . This being 

 so, it is evident that the primitive micromere in this case 

 contains a part only, of the entire epiblastic material. 

 The cocoons were found at various dates ranging from 

 March 2nd to April 18th. (PL XIV, figs. 1—11). 



II. Development of Arenicola marina'. — The ova are 

 deposited in vast numbers within a grape-shaped gelatinous 

 matrix furnished with an anchoring stalk as described. 

 The mass is delicate ulva-green in hue due to the colour- 

 ation of the contained ova, the matrix being transparent 

 and colourless. In size the masses vary from 2 to 2"5 cm. 

 in length by 1* 75 to 2 cm. in breadth. The ova are spherical 

 and 0'08 mm. in diameter. Segmentation is unequal 

 and a cephalotrochous embryo is the result. I was 

 anxious to watch the stages of segmentation but beyond 

 the bare fact noted nothing could be made out, owing to 

 the opacity of the vitellus. 



To describe the embryo (PL XIV, figs. 12—21) :— The 

 first appearance of cilia is in the form of an equatorial zone, 

 and a tuft of long cilia at the anterior pole or apex of what 

 eventually develops into the praeoral segment. With the 

 advent of these organs the embryo begins to rotate slowly 

 within the gelatinous matrix of the cocoon ; and shortly 

 two reniform pinkish eye-spots appear on opposite sides 

 above the zonal band of cilia : soon after this the embryo 

 become free-swimming. Either upon liberation or it may 

 be a little prior thereto, a ciliated pit— which eventually 

 becomes the mouth — makes its appearance a little behind 

 the zone of cilia, and this again becomes connected with the 

 posterior pole by a band of short cilia. In this advanced 

 stage — the latest I was able to observe — it is noteworthy 

 that what was the apical tuft of cilia in the first stage has 

 changed its position, moving a little distance ventrally ; 

 one of the cilia of this tuft has become extremely elongated 



