INTRODUCTION. SIX 



internally by a single layer of hepatic cells. Most of the 

 muscular fibres of the intestine are longitudinal, but near the 

 anus there are two distinct layers — -the external layer circular, 

 and the internal longitudinal. The epithelium of the anterior 

 portion of the intestine consists of short, thick, densely granular 

 cells ; in the posterior portion the cells are longer and their 

 contents less coarsely granular. A pair of tubular glands — the 

 rectal glands — open into the rectum or hinder portion of the 

 intestine ; and a single gland — the anal gland — situated in the 

 telson, opens immediately in front 'of the anus. The whole of 

 the alimentary canal is surrounded by a complete investment of 

 what is called the "fatty body," — which consists entirely of 

 rounded or polygonal nucleated cells. 



The heart of the Sand-hopper and its allies is elongated and 

 tubular, extending from the head to near the hinder extremity of 

 the pereion. At each end and usually from the sides arteries 

 are given off, and there are several pairs of apertures (venous, 

 ostiaj by means of which the blood enters the heart from the 

 surrounding blood-containing space — the pericardial sinus. 



The nervous system consists of a cephalic ganglion situated in 

 the head, of a chain of seven ganglia in the thorax, the most 

 anterior of them being connected with the cerebral ganglion by 

 commissures which embrace the gullet or stomach, and of three 

 abdominal ganglia. 



The organs of sense consist (1) of auditory hairs situated on 

 the basal joint of the upper antennae, (2) of sensory (tactile ?) 

 setae on the last and penultimate joints of the lower antennas, 

 (3) of the calceoli or "slipper-shaped organs," (olfactory organs) 

 situated on the flagella of one or both pairs of antennas, (4) 

 certain sensory setse occurring on the anterior margin of the 

 second pair of maxillae, and (5) of the eyes. 



The ovaries are two cylindrical sacs, situated one on each side 

 of the alimentary canal, and the oviducts open on the under 

 surface of the fifth thoracic segment. The testes are narrow 

 tubes opening externally on the under surface of the seventh 

 thoracic segment. 



The young of the Sand-hopper go through no metamorphosis 

 in the course of their development, but when they emerge from 

 the egg differ little save in size from the full-grown animal. 



