1907.] NATURAL SCIENCES OP PHILADELPHIA. 137" 



tion of the head, penetrating the brain in their course. The third pair 

 is dorso-lateral in position and also runs from the anus to the head. 

 Fibres from the second pair (ventro-lateral muscles) meet and cross in 

 the head cavity. The second set of muscles comprises muscle fibres 

 which traverse the head cavity in a dorso-ventral direction, from side 

 to side, and also obliquely. 



Nervous System. — Central Nervous System. — The central nervous 

 system (fig. 1) includes a simple brain, in close contact with the hypo- 

 dermis, circumoesophageal connectives, and a pair of ventral nerve 

 cords. The latter traverse the whole length of the trunk and are 

 throughout embedded in the hypodermis. They are connected Iw 

 transverse commissures, three being present in the second trunk seg- 

 ment, and two in each of the three following. A preoral transverse 

 commissure is also present, which appears to be the homologue of the 

 ventral portion of the nerve-ring of the trochophore larva of the an- 

 nelids. Ganglion cells are present, also embedded in the hypodermis,. 

 and arranged to form four well-defined pairs of ganglia. A fifth is 

 possibly indicated. Large nerve-trunks are given off from the cir- 

 cumoesophageal commissm-e to the oesophagus and the ventro-Iatcral 

 muscles. 



Sense Organs. — The eyes consist each of a bilobed lens and a pigment 

 cup. The inner end of the latter is buried in the outer surface of the 

 brain. The large sense hairs on the anterior surface of the head are 

 connected with the brain by sensory neurons. Problematical sense 

 organs are found on the sides of the neck. 



Alimentary Canal. — The mouth is triradiate and is situated on 

 the ventral side of the head. It opens into an oesophagus which passes 

 upward, then backward for a short distance, to empty into a capacious 

 stomach which fills the body cavity in the third and fourth trunk seg- 

 ments. Near the anterior border of the fifth trunk segment the 

 stomach joins the intestine, which passes in an almost straight course 

 to the anus. The latter is situated dorsad of the caudal appendage.. 

 The alimentary canal is lined throughout with long cilia. The oeso{)h- 

 agus is provided Avith a large proboscis (fig. 23, pro.). The latter is 

 in shape ovoid, flattened dorso-ventrally, and is situated just within 

 the mouth, in an involution of the posterior wall of the ascending limb 

 of the oesophagus, and is attached to the oesophageal wall at the bot- 

 tom of the involution. The proljoscis is made up of flat cells arranged 

 like coins in a pile, and separated by chitonized partitions. On each 

 side of every partition is a layer of muscular fibres. The function of 

 the latter appears to be that of elongating the proboscis. This organ, 

 is moreover provided with a set of extrinsic muscles. 



