144 THE LANCELET 



forceps, so as to expose the alimentary canal, notochord, and other parts 

 entirely. Note and draw the following structures : — 



1. The Skeletal System. 



(a) The notochord, a conspicuous flexible rod along the whole length 



of the body, between the dorsal spinal cord and the alimentary 

 canal. 



(b) The skeleton of the dorsal fin, a single series of fin-rays of connective 



tissue, extending along the dorsal surface. 



(c) The skeleton of the ventral or anal fin, a double series of fin-rays of 



connective tissue, extending from the atrial aperture to the anus. 



(d) The buccal cartilage, supporting the buccal cirri, around the oval margin 



of the oral hood. 



(e) The skeleton of the gill-slit region of the pharynx (see the Alimentary 



System). 



2. The Alimentary System. 



The buccal cavity (stomodaeum) is lined with cilia, a part of this cihated 

 lining having specially long cilia which form the " wheel-organ," which helps to 

 drive water containing food particles into the pharjoix. The buccal cavity 

 leads into the pharynx, and separating them is a muscular diaphragm, the 

 velum. The central opening of the velum, the mouth, has a fringe of twelve 

 tentacles projecting into the pharjoix. The pharynx is large ; it occupies most 

 of the atrial chamber, and forms about half of the alimentary canal. The atrial 

 cavity or atrium is a big space completely surrounding the anterior two-thirds 

 of the alimentary canal, except dorsally. The side walls of the pharynx are 

 perforated by a large ntunber of gill-slits obliquely directed, downwards and 

 backwards. Along the mid-ventral wall of the pharynx is a deep groove, the 

 endostyle, lined with ciliated and glandular epithelium. Along the mid-dorsal 

 wall there is a deep and ciliated epipharyngeal or epibranchial groove opening 

 into the stomach. Following the pharynx is the equally long and straight 

 intestine, which gradually narrows towards the posterior end, where it ends in 

 the anus. The anterior part of the intestine is somewhat swollen and forms 

 the "stomach." A pouch-like outgrowth from the floor of the "stomach" 

 is the liver, which extends forwards on the right side. 



Remove a small portion of the pharynx (the hind end), mount it in glycerine 

 on a slide, examine under the microscope, and note :- — 



Between the successive oblique gill-shts are supporting skeletal structures 

 known as the gill-arches, each strengthened by a chitinous axial rod, and each 

 containing a coelomic cavity. These rods are of two kinds and are arranged 

 alternately : — 



(a) The primary rods, split or forked at their ventral ends. 



(fe) The secondary rods or tongue-bars, unsplit, not forked. Each gill-slit, 



originally single, becomes double by the later development of these 



unsplit rods. 



There are short horizontal bars connecting the successive pairs of primary 

 rods together, while dorsally all the rods are connected by a horizontal rod. 



