502 W. JR. Coe — Anatomy of Cerehratulus lacteus. 



muscular fibres, while the inner border of the fibrous core abuts 

 closely against the circular muscular layer of the body-wall. Out- 

 side the ganglion-layer is a loose and irregular sheath of connective 

 tissue {cs'). From the ganglion-layer bundles of nerve-fibres run 

 both into the fibrous core and into the nerve-plexus to be dis- 

 tributed throughout the body. 



The nerve-plexus lacks ganglion-cells but contains, besides nerve- 

 fibres, an abundance of nucleated connective tissue fibres. It appar- 

 ently serves as the path through which the nerves from the lateral 

 cords are distributed to the various organs of the body. 



The Cephalic Slits and Side- organs. 



The side-organs, posterior brain-lobes, or olfactory lobes, as they 

 are variously called, are a pair of large, spherical organs lying in the 

 cephalic blood-lacunae, closely pressed against the posterior borders 

 of the dorsal brain-lobes (Plate XI, fig. 2, so). Each has a thin 

 external sheath of connective tissue and a delicate outer endothelium 

 lining the blood-lacuna. Except where they are united with the tis- 

 sues of the head in front and on the anterior ventral border, they are 

 surrounded with blood. This fact led the observers of a few years 

 ago to consider the side-organs as having a respiratory function. A 

 large branch from the fibrous core of the dorsal brain-lobe passes 

 directly into the middle of these organs, and bent about in their tis- 

 sues lies a canal with sensory epithelium. This canal opens to the 

 exterior of the body near the posterior ends of the cephalic slits, and 

 places these organs in direct communication with the water in which 

 the animals live. Their function is still somewhat problematical, 

 though it is undoubtedly sensory, and is most generally believed to 

 be olfactory. 



" The lateral cephalic slits (Plate X, figs. 1, 2, 3, 8, Is) are large 

 and deep, extending the entire length of the head and running for- 

 ward close to and a little above the proboscis-pore, those of opposite 

 sides not uniting together except by a very shallow furrow. . . . 

 Their margins are thin and mobile, often undulated or curled back so 

 as to open the slits widely and expose the deep posterior pits. . . . "* 



These slits are furnished with an epithelium which is not very dif- 

 ferent from that of the rest of the integument, except that the 

 cilia are longer and more conspicuous. Among the bases of the elon- 

 gated cells are, especially in the posterior portions of the slits, many 



* Verrill, (1) p. 434. 



