X INTRODUCTION. 



appendages are the stalked eyes, the stalk or peduncle of which 

 consists of a short hasal portion the basiophthahnitc, and a 

 larger terminal portion the podophthalmite bearing the eye at 

 its extremity. 



The body of the Prawn is thus seen to consist of twenty 

 segments or somites, six belonging to the head, eight to the 

 thorax, and six to the abdomen. 



The breathing organs of the Prawn consist of gills or "bronchia 

 situated in the cavities enclosed on either side of the thorax by 

 the branchiostegites or lateral folds of the carapace. These 

 branchiae in the Prawn are of the kind termed trichobranchice, 

 i.e., they consist of a central stem, from which are given off 

 numerous lateral filaments. They are twenty in number 

 altogether in the Prawn, exclusive of the branchial epipodites 

 already mentioned as connected with the bases of the legs. 

 Two of the gills in each thoracic segment, except the first and 

 last, are attached to the membrane between the sternum and 

 the coxa of the appendage ; the first thoracic appendage has 

 one gill in that position, and the last none ; one gill is united to 

 the surface of the pleuron above the base of each of the thoracic 

 limbs except the first. 



The organs of circulation of the Prawn consist of a heart and 

 blood-vessels. The heart is a polygonal sac situated in an 

 enclosed cavity the pericardial sinus immediately below the 

 carapace ; it receives from the pericardial sinus through certain 

 slits in its walls the blood brought back from the gills by vessels 

 called the branchio-cardiac vessels. The blood thus returned to 

 the heart from the gills is propelled to the various tissues and 

 organs of the body by six arterial trunks, five anterior and one 

 posterior, and eventually finds its way into large irregular ill- 

 defined vessels or sinuses, from which it is conveyed to the gills 

 by the afferent branchial vessels. 



The organs of digestion of the Prawn consist of an abmentary 

 canal, divided into oesophagus, stomach, and intestine, and 

 extending through nearly the whole length of the body from the 

 mouth to the under side of the telson, where the anus is situated. 

 The mouth is a slit, opening into a wide tube the oesophagus, 

 which passes nearly directly upwards to enter a tolerably spacious 



