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CHAPTER XIV 



MORPHOLOGY OF THE SHIPWORM 



The anatomical structure of the shipwornis is extremely interesting, both from 

 the point of view of the coordination of structure and function, and from that of 

 comparison with related types which live under less unusual conditions. On superficial 

 examination, Teredo would appear to bear hardh- a remote resemblance to such a form 

 as the common soft shell clam, Mya. But on dissection it is found that the organ 

 sN'stems and their relations in the two forms are essentially the same, and the families 

 Myidae and Teredinidae are placed in adjoining sub-orders in modern systems of 

 classification. 



As compared with Mya, the body of Teredo is greatly elongated, the visceral 

 mass occupying approximately the anterior fourth of its length, and the gills the 

 remainder. The small, nearly globular, bivalve shell is much reduced, so that it covers 

 only a small portion of the anterior part of the body. In comparison with less special- 

 ized lamellibranchs, and indeed from a consideration of its own earh- development, 

 Teredo may be regarded as having greatly outgrown its shell posteriorly. Most of the 

 organs therefore lie back of the posterior adductor muscle, instead of anterior to it 

 as in other bivalves. 



Although thus out ot their normal positions, the organ systems nevertheless 

 maintain in considerable degree their original relations. The kidneys, heart and peri- 

 cardium ha\e been elongated and rotated about the posterior adductor muscle through 

 an arc of 180 degrees, so that their morphologically posterior end becomes anterior, 

 and their dorsal side ventral. This places the kidneys above the pericardium, and the 

 renal orifices at the posterior end of the viscera. The genital orifices have followed the 

 renal orifices posteriorly, but the gonads cannot be regarded as having taken part in 

 the rotation about the posterior adductor, as they still maintain a position ventral to 

 the pericardium. The visceral ganglion has moved from its position beneath the pos- 

 terior adductor muscle to the posterior tip of the visceral mass. The cerebral and pedal 

 ganglia and their connectives form the usual ring about the oesophagus. 



The mouth and anus retain their usual positions, the former between the anterior 

 adductor muscle and the foot, and the latter on the dorsal side of the posterior adduc- 

 tor. The oesophagus is the t\'pical short, flat ciliated tube. The stomach, however, is 

 greatly elongated and relati\ely large. Its capacity is further augmented by a long, 

 cylindrical caecum opening from its posterior end. The usual crystalline style and its 

 mechanism are well developed, the sac of the style lying in the small, discoidal foot 

 of the animal. The digestive glands are large and are closely applied to the stomach. 

 The intestine, which in Mya is coiled upon itself, may be regarded in Teredo as having 

 been uncoiled and extended posteriorly in a long loop about the caecum. It does not 

 pass through the heart, as in M\-a, nor through the pericardium. The anus opens 

 into a long anterior diverticulum of the suprabranchial ca\-ity, the anal canal. More 

 detailed comparisons with the typical lamellibranch structure will be made as the 

 various systems are considered. 



The following account is based primiU'ily on the anatomy of Teredo iiuvulis, with 

 occasional reference to what is known of other species of shipworm. 



Anteriorly the gills become separated by the visceral mass, and each continues 

 forward as a mere rudiment lo the region of the mouth. Here each again spreads out 



