Dissected so as to show the nervous, circulatory and digestive systems in situ, and in the relations they hold to each other and to the external body walls.
THE animal has been bisected longitudinally and the left half of the body removed together with its appendages, their muscles, the left lobes of the liver, and green gland. The series of nerve ganglia and connecting commissures lies above the sternal elements of the various somites, the heart beneath the central area of the omostegite, whilst the reproductive and digestive organs occupy a position midway. The appendages of the right side are left in situ. The modified first and second pairs of abdominal limbs show that the specimen belongs to the male sex.
The supra-oesophageal and twelve post-oral ganglia, six thoracic and six abdominal, are seen in profile. The great length of the commissures between the first-named ganglion and the first of the post-oral series or infra-oesophageal ganglion, whence the six pairs of appendages connected with the mouth are innervated, may be noted as corresponding with the elongated antennary sternum. In the abdominal series a slip of blue paper has been placed beneath the commissures uniting the fifth to the sixth ganglion. A white bristle has been introduced through the mouth into the oesophagus and stomach. This latter organ is large and extends forwards behind the eyes, but its anterior wall has been displaced a little backwards. It is divisible into a larger cardiac and a smaller pyloric portion, the pyloric situate just in front of a piece of blue paper placed under the hepatic artery. The lower edges of the liver lobes of the right side may be seen below the posterior cardiac region and extending backwards below the commencement of the intestine. The aperture of the common duct of the left liver lobes lies at the apparent termination of the hepatic artery, a projection in front of it marking the so-called pylorus.
The part of the digestive tract into which the liver ducts open constitutes the mesenteron, and represents the archenteron of the embryo. The oesophagus and stomach are differentiations of the stomodaeum, whilst the intestine, which runs straight to the anus on the ventral surface of the telson, and is of great relative length, is formed from the proctodaeum of the embryo. The mesenteron and its appended glands are lined by endoderm, the oesophagus, stomach and intestine by ectoderm which secretes a chitinous cuticle. This cuticle is cast by the animal whenever it moults.
The heart is a rounded body lying below the cardiac area of the omostegite. Of the arteries, to which it gives origin, the left hepatic passes obliquely downwards in front to gain the liver lobes, the sternal artery obliquely downwards behind to pass between the commissures uniting the third and fourth thoracic ganglia. Both these arteries have pieces of blue paper placed beneath them. Another piece of blue paper has been placed under the superior abdominal artery in the region of the fourth abdominal tergum. The left anterior lobe of the testis lies below and in front of the heart: the posterior azygos lobe beneath and behind it, whilst numerous coils of the right vas deferens come into view between the testis and intestine. The powerful and complex flexor muscle of the abdomen may be seen below the intestine and above the nerve chain extending into the thorax to be attached to the endophragmal skeleton (see ante, p. 170) as far forwards as the oesophagus. It is by means of this muscle, by which the abdomen is suddenly flexed, that the animal executes its rapid backward darts through the water.
The ambulatory thoracic limbs are employed for the slower movements of crawling; and the muscles that move these limbs on the right side may be seen passing through the intervals of the endophragmal skeleton from their origins upon the epimera displayed in Preparation 36.