GENERAL HISTOLOGY

Detailed explanation-1: -Vertebrates are all chordates that have a backbone. The other two subphyla are invertebrate chordates that lack a backbone. Members of the subphylum Urochordata are tunicates (also called sea squirts). Members of the subphylum Cephalochordata are lancelets.

Detailed explanation-2: -Sea squirts belong to the phylum Chordata, which includes all animals with a spinal chord, a supporting notochord (backbone), and gill slits at one point in their lives–everything from fish to humans.

Detailed explanation-3: -INTRODUCTION. Cephalochordates, commonly called amphioxus or lancelets, are marine invertebrate chordates. They were traditionally held as the closest living relatives of vertebrates and have long occupied a key phylogenetic position in our discussions on the origin of vertebrates.

Detailed explanation-4: -Tunicates and lancelets are examples of invertebrate chordates since they have a notochord, but it does not develop into a vertebral column. Pharyngeal slits are in the pharynx, the region of the digestive tract just behind the mouth (Fig. 3.98).

Detailed explanation-5: -sea squirt, also called ascidian, any member of the invertebrate class Ascidiacea (subphylum Urochordata, also called Tunicata), marine animals with some primitive vertebrate features.