12 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



in the body wall. In closing, the rim of the siphons is thrown into folds, whose 

 number and arrangement varies with different species. 



The outer layer of the body wall consists of a thick, tough, sometimes some- 

 what gelatinous, often warty, covering, the tunic or test, which is secreted by 

 the ectoderm beneath it. In some tunicates (dona, Corella) the test is so trans- 

 parent that practically all of the internal organs are visible through it. The 

 test is attached to the underlying body only at the siphons. 



Draw the animal. 



2. Dissection of a tunicate. (A demonstration dissection may be substituted 

 for individual dissection by the student.) As different tunicates vary in the 

 details of their internal structure, the following account is of necessity somewhat 

 generalized. It is based chiefly on the anatomy of Ciona. Make a cut through 

 the base of the tunic, avoiding injury to the underlying wall, and peel off the 

 tunic. Note its attachments at the siphons, and cut through these attachments, 

 discarding the tunic. The soft body wall or mantle is thus revealed, the tunic 

 being, as already explained, a secretion of the outer layer of the mantle. The 

 muscle fibers which operate the siphons may be visible as stripes in the mantle; 

 they are conspicuous in Ciona. 



Fasten the animal in a wax-bottomed dissecting pan by pins through the 

 rims of the siphons and through the extreme basal end. Fill the pan with water 

 to cover the animal. 



The mantle incloses the viscera which tend to adhere to it, so that its removal 

 without injury to the viscera is difficult. In transparent forms the viscera may 

 be studied through the mantle. In case it is necessary to remove the mantle, 

 make a longitudinal slit in it from the atrial siphon to the basal end and pull it 

 off in small strips, separating each strip gently from the underlying parts. The 

 principal internal parts are the following. The oral siphon leads into a large 

 thin-walled bag, which in some forms, as Ciona, extends the length of the body. 

 This bag is the pharynx. Its wall appears to the naked eye like mosquito netting, 

 because it is pierced by numerous minute openings, the gill slits or visceral clefts. 

 The cavity outside of the pharynx is the atrium; it communicates with the 

 exterior by way of the atrial siphon. The lower end of the pharynx narrows 

 abruptly into a short esophagus which opens into a widened stomach, situated 

 below or to one side of the basal end of the pharynx. The stomach is curved 

 so that its long axis lies at right angles to the long axis of the pharynx. In some 

 cases (Molgula) brown digestive glands can be seen covering the stomach. The 

 stomach leads into an intestine which immediately makes a loop, doubling 

 back so as to lie parallel to the stomach. It then bends and extends straight 

 upward toward the atrial siphon, terminating by an anus situated within the 

 atrial cavity. 



The reproductive organs consist of a single or paired mass, each being her- 

 maphroditic, that is, composed in part of an ovary and in part of a testis. This 

 hermaphroditic gonad is located either in the space between the intestinal loop 



