400 INVERTEBRATA CHAP. 



Shortly after the foundation of the apical plate, the so-called 

 dorsal organ is formed on the anterior aspect of the aboral pole of 

 the larva. It develops in exactly the same manner as the apical 

 plate, that is, as an invaginated plate of cylindrical cells, which, how- 

 ever, is from the beginning markedly bilaterally symmetrical. The 

 cavity of the invagination becomes tube-like and its walls are formed 

 by a single layer of cubical ciliated cells, but its noor becomes con- 

 verted into a bilobed ganglionic mass which is connected with the 

 apical plate by two strands of nerve fibrils. This ganglionic mass 

 may be compared to the brain or cerebral ganglia of Annelida 

 and Mollusca, whilst the pit, which remains as dorsal organ, may be 

 compared to the cerebral pit of Molluscan larvae. 



Whilst these changes have been going on the blastopore has 

 become completely closed and the archenteron separated from the 

 ectoderm ; the ventral ectoderm has become thickened, and it con- 

 sists in fact of a single layer of cylindrical ciliated epithelium. 



On this surface now appear three invaginations, one behind the 

 other. The front one gives rise to the long stomodaeum or 

 oesophagus, the cells lining which are all ciliated. The mouth is 

 surrounded by specially long cilia. The stomodaeum joins the sac- 

 like archenterou, fuses with it, and opens into it. 



The hindermost invagination gives rise to the proctodaeum, the 

 cells lining which carry short cilia. It is borne on a projecting portion 

 of the ventral surface known as the anal cone, which, as we shall see, 

 may be compared to the post-trochal portion of a Trochophore larva. 

 The archeuteron becomes constricted into a main sac-like portion 

 which forms the stomach, on the lower wall of which glandular 

 cells are developed, and a small bud-like intestine which meets and 

 fuses with the ectoderrual rectum, and so the alimentary canal is 

 completed. 



The central invagination gives rise to the atrium or vestibule, 

 and is shallower than the others. Its outline is at first round but 

 later becomes square. Its front wall, which abuts on the hinder 

 wall of the oesophagus, develops into a ventral prominence known as 

 the epistome ; and on this is a bilobed thickening, which is connected 

 round the oesophagus with the dorsal organ by two strands of nerve 

 fibres, and which will develop into the main nerve ganglion of the 

 adult. 



At the sides of the atrial cavity are a pair of somewhat similar 

 thickenings, and on its hinder wall, where it touches the rectum, is 

 a third bilobed thickening. The lateral thickenings are obviously 

 ganglionic in character, for they become connected with the dorsal 

 organ, or, as we may now term it, the cerebral ganglion, by nervous 

 strands. The posterior thickening is the transitory rudiment of 

 a ganglion which disappears during embryonic life. 



The edge of the atriuni carries a row of specially powerful cilia 

 which subserve locomotion and correspond to the corona of the Ecto- 

 procta. These cilia are everted when the embryo escapes and becomes 



