IX DEVELOPMENT 245 



convey the spermatozoa to the exterior; but in the female the body- 

 wall ruptures to allow the ova to escape, and then the animal dies. 

 The development of Polygordius has been made the subject of 

 very careful study ; the larva has long been known, and is a 

 typical " trochosphere " of rather a depressed form. This 

 " trochosphere " larva is of considerable importance, as it makes 

 its appearance in sundry groups of animals in some form or 

 another. Here, in Polygordius, it has the appearance of a couple 

 of wide but low cones united together by their bases, which form 

 the equator of the larva. This equator carries a double girdle 

 of cilia, dividing the animal into a preoral and postoral region ; 

 for the mouth is placed on one side of the animal between the two 

 girdles, while the anus lies at the apex of the postoral cone, and 

 is surrounded by another girdle of cilia. The alimentary canal is 

 divisible into three regions ; it is separated from the body- wall by 

 an extensive space, which contains cells destined to give rise to 

 muscles and nephridia. A nervous system (apical plate) is present 

 at the apex of the preoral cone. This little larva swims freely 

 on the surface of the sea, moving, balancing, and feeding by 

 means of the girdle of cilia. It soon increases in length by the 

 active growth of the apex of the postoral cone, which becomes 

 cylindrical and then segmented externally and internally. The 

 greater part of the original larva remains of the same shape as 

 before, and forms the head (prostomium and peristomium) : small 

 tentacles grow out of the preoral lobe, and after a gradual 

 reduction in the relative size of the " head " by the growth of 

 the segmented " body," the animal becomes worm-like and develops 

 into a Polygordius} 



Order II. Polychaeta. 



Anatomy of Nereis. In order to obtain a general idea of 

 a Polychaete worm, it is well to study a concrete example, and 

 for this purpose the common Nereis serves excellently. Several 

 species (see p. 315) occur more or less commonly on our coasts, 

 and the general remarks will apply to one as well as to another. 



Nereis pelagica Linnaeus reaches a length of 5 to 6 inches, and 

 is about ^ inch across. It is convex above, nearly flat below. Its 



1 T. J. Parker, Lessons in Elementary Biology, London, 1891, p. 267, gives a full 

 account of the anatomy and development of Polygordius. 



