STRUCTURE 477 



in the latter. So in the sea-squirt one of the Vertebrate 

 or Chordate features is the ventral position of the heart. In 

 Ascidia it lies on the ventral and posterior edge of the stomach, 

 projecting into a space (called the pericardium) which is part 

 of the original body-cavity. It is the simplest of all hearts 

 — a spindle-shaped contractible tube (with the only striped 

 muscle in the body) — and it has the remarkable peculiarity that 

 it drives the blood out first at one end and then at the other. 

 There is at brief intervals a reversal of the circulation. We 

 cannot go into the details of this, but we venture to quote from 

 one of the greatest authorities on Tunicates a description of the 

 striking phenomenon of " reversal ". Professor Herdman 

 writes: "If a small or young Ascidia be placed alive, left side 

 uppermost, in a watch-glass or small trough of sea water, and 

 examined with a low power of the microscope, the heart will 

 be readily seen near the posterior end of the transparent body. 

 It will be noticed that the ' beating' looks like successive waves 

 of blood pressed through the tubular heart from one end to the 

 other by its contractions. After watching the waves passing, 

 let us say, from the right-hand end of the heart to the left for 

 about a minute and a half (perhaps 60 or 80 to 100 beats), it 

 will be seen that they gradually become slower and then stop 

 altogether. But after seven or eight seconds a faint wave of 

 contraction will start from the left end of the heart and pass 

 over it to the right ; and this will be followed by larger ones for 

 a minute and a half, and then again a pause will occur and the 

 direction change." It is supposed that the heart works too 

 energetically for the circulation ; the blood cannot get quickly 

 enough through the fine channels in the branchial sac and 

 viscera and back pressure results. It need hardly be said that 

 there are no valves. The blood usually appears colourless ex- 

 cept under the microscope when it is seen to contain various 

 coloured corpuscles as well as uncoloured. 



We close our brief description of a common sea-squirt with 

 a reference to the reproductive organs or gonads. The Ascidian 

 has both an ovary and a testis ; in other words, it shows herma- 

 phroditism. The gonads lie in the loop of the food-canal, on 

 the left side of the body, and the testes spread over the surface 

 of the ovary. There is an oviduct for the passage of the ova 

 to the peribranchial cavity and thus to the exterior, and running 



