438 THE ANIMAL KINGDOM. 



The epidermis consists of a single layer of columnar cells, ciliated in 

 the larva, with interspersed sense-cells furnished each with a stiff projecting 

 hair, and continuous basally with a nerve-fibre. The cutis is transparent 

 and non-nucleated. The sub-cutaneous tissue contains a remarkable 

 system of anastomosing (lymph ?) tubes, lined by an endothelium, and 

 connected with two ventrally-placed tubes running longitudinally one in 

 each metapleure. The notochord extends nearly from one end of the 

 body to the other, is pointed at each extremity, and consists of a series of 

 transverse discs with a dorsal furrow containing a retiform tissue, the whole 

 enveloped in two sheaths. The outer sheath sends out dorsal laminae, 

 which inclose the spinal cord ; and ventral laminae, which extend into the 

 walls of the body and the epipleures (infra). These laminae are continuous 

 with the sheaths (myocommata) of the myomeres both of the epipleures 

 and of the body behind the branchial region. The myomeres, 62 in number 

 in A. lanceolatus, are arranged on each side in a continuous series. The 

 dorsal and ventral sections of each myocomma which are not separated 

 organically from one another, meet at an angle pointing forwards. There is 

 also a small and remarkable set of transverse ventral muscles. The muscles 

 are striated rhombic plates devoid of sarcolemma. The nervous system 

 consists of a spinal cord with a central canal, both of which swell out 

 anteriorly. The anterior enlargement probably corresponds to the hind- 

 and mid-brain with that part of the fore-brain which extends dorsally as 

 pineal gland (?), a structure represented by the so-called olfactory nerve, 

 which is really a tube opening externally in a ciliated depression. The tube 

 represents an aperture left when the neural plate folds over in development 

 to form the neural tube or canalis centralis. The ciliated depression is at 

 first median and dorsal, and afterwards shifts to the left side. Three pairs 

 of nerves arise from the anterior enlargement. The spinal nerves possess 

 dorsal and ventral roots which are not connected outside the cord (Rohon). 

 The ventral roots are purely motor, and arise as short columns. The 

 dorsal are the principal and, according to most authorities, the only, spinal 

 nerves. Their roots possess no ganglia, and the right and left series 

 alternate more or less one with another. The grey matter consists only of 

 ganglion cells. A sac, containing sense-cells with refractile hairs, opens on 

 the dorsal wall of the oral cavity in the centre of a disc of ciliated cells. It 

 is probably an organ of smell or taste, and is derived from the left anterior 

 entero-coelic pouch of the larva (infra). It is doubtful whether the pig- 

 ment speck at the anterior end of the brain can be considered as an eye. 



The mouth is a somewhat oblique ventral slit surrounded by twelve 

 cirrhi, which are supported by rods borne upon a ring of twelve pieces, and 

 covered by papillae rich in sense-cells. The oral cavity, produced by a 

 forward growth of the epipleures, is separated from the pharynx by a free 

 fold or velum with fringed edges and many sense-cells. The velum can be 



