Monday, January 21, 2008
Shell Beach Invertebrates
Baby sea star
Tunicates - Colonial Sea Squirts
Sea squirts / Sea Pork are an early stage in the evolution of the chordates or animals. There are three types of tunicates: solitary, colonial, and compound. All have two siphons, one for intake and the other for expelling wastes. Colonial tunicates reproduce by budding. Sea squirts have a primitive notochord (vertebral column, backbone) called a urochord(lack segmentation throughout the body and tail) and are distantly related to fish, whales, and humans.
Maine gastropods (snails) reproduce dioeciously (male and female individuals). Egg deposition is in masses surrounded by a capsule, which is usually attached to the substratum. Most larvae develop into a free-swimming veliger. The characteristic feature of the veliger is the swimming organ called a velum, which consists of two large semicircular lobes bearing long cilia. The shell develops spirally in the veliger and may remain at the apex of the adult shell for some time. In sea slugs a shell appears in the veliger and is later cast off during metamorphosis.