GENERAL CONSIDERATIONS. 259 



Regarding the fragmentary knowledge we have of the embryonic develop- 

 ment of the Solenogastres and the light which this throws on the question of 

 the primitive characters of the group, it must be admitted that very little con- 

 clusive evidence has appeared. In an earlier paper (Heath, 1911) it was shown 

 that the Chiton and annelid in their development up to the trochophore stage 

 follow practically the same course. The great test or modified velum in the 

 Solenogastre larva cannot therefore be looked upon as a primitive mark; nor 

 can the posterior invagination which may represent a dorso-terminal sense organ 

 against which the nerve band abuts. Unfortunately we have no conclusive 

 evidence regarding the presence of a true shell nor any data relating to the form 

 and size of the foot. Whether adult characters are to be considered primitive 

 or secondarily modified must accordingly largely rest upon comparative ana- 

 tomical studies and the personal factor in interpreting such evidence. 



As Nierstrasz has maintained the various species of Solenogastres show a 

 truly surprising amount of variation, and while it is possible to find a fundamental 

 plan upon which all are constructed it is most difficult to decide which features 

 are the most primitive. Personally I am strongly of the belief that the ancestral 

 Solenogastre was provided with a mantle cavity containing a pair of ctenidia 

 and the openings of the coelomoducts and digestive tract, and a creeping sur- 

 face or foot provided with two sets of glands. Whether a shell was present 

 or absent cannot be decided. The digestive tract was pro\dded with a typical 

 radula, dorsal and ventral salivary glands, while the mid-gut lacked a clearly 

 defined digestive gland. The heart, in the posterior end of the body, communi- 

 cated on one hand with a sinus from the ctenidia and in the other direction 

 connected with the dorsal aorta, which supplied the gonad and opened through 

 a septum limiting the head cavity. This septum was also perforated ventrally 

 to allow the flow of blood into the visceral cavity and probably a ventral sinus 

 from which it passed to the ctenidia. The coelom comprised a genital section 

 opening into the pericardium, which in turn communicated with the exterior 

 by means of two simple, distinct coelomoducts. The nervous system, having 

 essentially the same configuration as it now possesses, was probably more 

 diffuse. 



If such indeed does represent the general plan of the ancestral Solenogastre 

 then it follows that the members of the Chaetodermatidae are highly modified 

 in most respects. They have retained their ctenidia, their relatively simple 

 coelomoducts, and in Limifossor there is an anterior septum and a well-developed 

 radula. In the neomenians the foot and glands still persist, and in certain 



