120 



INVEETEBKATA 



CHAP. 



mes.ect 



cells here are somewhat larger than elsewhere. The whole shape of 

 the blastula now resembled an obtuse cone. 



Two large cells are budded into the cavity of the blastula, or 

 blastocoele, one at each side of the flattened surface. These attach 

 themselves by pseudopodia to various portions of the blastula wall. 

 Then a patch of cells lying in the centre of the flattened surface is 

 invaginated and forms a sac-like gut, whilst at the apex of the cone 

 a thickening is formed which becomes slightly invaginated and forms 

 a saucer -shaped depression. The cells forming this depression 



develop long stiff cilia, and in this 

 way a characteristic apical sense- 

 organ is formed. The blastula has 

 thus become a gastrula (Fig. 94). 



Each of the two large colls which 

 passed into the blastocoele divides so 

 as to give rise to a mass of branched 

 cells, and these cells put out long 

 pseudopodia which attach them- 

 selves to gut, to apical organ, and to 

 skin, and some of these pseudopodia 

 become converted into muscle fibres. 

 Such cells are termed mesenchyme. 

 The gut becomes differentiated into 

 a sac -like globular stomach and a 

 funnel-like oesophagus. 



Eound the edge of the flattened 

 surface there is differentiated a 

 thickened band of cells carrying 

 specially long cilia. This band is 



called the " prototroch," and it becomes the sole locomotor organ of 

 the embryo, which now becomes a larva, escaping from the egg-shell 

 and swimming about. The shell is broken by a spiral boring move- 

 ment executed by the embryo. The prototroch is at first a simple 

 circle, but it grows out into two lateral, downwardly -directed 

 processes, like the ear-lappets of a policeman's helmet, and so the 

 characteristic form of the Pilidium larva is attained (Fig. 95). 



The principal muscles which have now been formed by the mesen- 

 chyme are as follows : The retractor of the apical plate is a band 

 of fibres which is attached above to the apical plate, and passes down- 

 wards splitting into right and left portions. These latter fibres are 

 at first attached to the gut, but later, when the lappets are formed, 

 they extend down into them, and their muscle fibrils extend from the 

 oesophagus to the stomach, and from the post-oral ectoderm to the 

 stomach ; there is a strong sphincter muscle round the mouth. 

 These muscles are all formed by processes of the larger mesenchyme 

 cells. The smaller mesenchyme cells apply themselves to the inner 

 surface of the ectoderm and to the outer surface of the gut. They 

 give rise to a series of so-called " peritoneal " muscles which take the 



FIG. 94. The young gastrula of Cere- 

 bratulus lacteus. (After C. Wilsoii.) 



op, apical plate ; end, gut ; mes.ect, mother 

 cell of mesenchyme. 



